MUMBAI, India, July 19 — Behram Harda was a dancer in the Bollywood films of the 1970s, gracing the screen with his twist and his cha-cha.

Then he became a rodent assassin.

Today, in the sprawling B Ward of this teeming, filthy, exhilarating city, Mr. Harda is admired by his colleagues as the last of the great Mumbai rat catchers. His is a dying breed in a city whose dreams of being rat-free recede year by year.

Mr. Harda, 55 years old with salt-and-pepper stubble, is a gentle, relentless executioner. He fumigates. He drops poison laced with garlic and chutney into burrows. He brings new traps to shopkeepers and collects the previous catch for killing. The rats are sometimes drowned in buckets. Other times they are seized by the tail and smashed onto the hot pavement.

These are the small, unsung tasks that keep big cities humming. Behind every great city there is, or perhaps ought to be, an obsessive, fearsome rat catcher, toiling silently so that bankers can bank, film stars film and vendors vend.

But Mr. Harda is an Indian Sisyphus. When he got the job 33 years ago, the rats were no match for the catchers. Government service attracted India’s brightest in those days, and Mumbai was still clean enough to starve rats of the garbage on which they snacked. But in three decades, India has turned inside out, and so has the equation between catchers and rats.

Private-sector jobs in call centers and software firms beckon, and the government struggles to attract men of Mr. Harda’s caliber. Many rat-catching posts lie vacant. Meanwhile, Mumbai has metastasized from a genteel city of a few million into a grimy megalopolis of 17 million. More than half of the population lives in shanties surrounded by garbage — and, consequently, by rats.

Strolling through a low-cost housing complex, Mr. Harda huffed at a heap of garbage defenestrated from the apartments above.

Eggshells. Butter wrappers. Banana peels. Coconut halves. Mango peels. Bread. And sure enough, in the midst of it all, rat burrows.

Mr. Harda accepts that his is a losing battle. In 10 years, he expects Mumbai to have more rats. “It is impossible to get them,” he said.

But he keeps trying.

To accompany Mr. Harda on his rounds of rat-infested areas is to navigate a parallel city, a world apart from the malls and luxury apartments sprouting in Mumbai. On these streets old ladies sweep human waste into drains, men soap and bathe themselves in the gutter and women pluck lice from the heads of their husbands and brothers.

Mr. Harda and three deputies strode through these lanes like Ghostbusters, cages in hand, nodding at passers-by for whom their arrival is a daily reassurance. They stopped at food warehouses full of sacks of rice, sugar and lentils. Many had installed cages the day before and found a specimen or two. Mr. Harda gathered the catches into a single, swarming cage.

By 10:05 a.m., they had two full cages in custody. Now the rats had to die.

The cages were dipped one by one into a bucket, but the bucket was too short and many of the rats managed to keep their noses above the water level. When the cage was restored to dry ground, the rats patiently rearranged their fur as if nothing had happened.

But Mr. Harda had an alternative plan, which was not subtle or hygienic but was terrifyingly effective. One of his deputies plucked the rats from the cage one by one and, with the vigor of a Whack-a-Mole player, slammed each one onto the ground. The rat would convulse with shock, then suddenly go still. In some cases, its limbs would gyrate, Elvis-like, for a final few seconds. A few especially resilient souls briefly resurrected themselves to make a last, death-defying jump. And then they, too, died.

The men killed 26 rats in five minutes. Afterward, a small fraction would be sent to a laboratory to be tested for bubonic plague.

All this may seem like strange toil for a man who once danced in hit Bollywood movies like “Brahmachari,” and who still looks, in a certain light, like a man of film, his graying hair slicked back with shiny cream.

But when he was a young dancer, Bollywood was not much of an industry, and a municipal job in a socialist country seemed more secure. His father made him trade cha-cha for civil service. “I killed all my ambitions,” he said.

How was his father to know that India, 17 years later, would swivel to capitalism, that Bollywood would grow into a cash machine, that government jobs would surrender their appeal?

Mr. Harda is by no means bitter. He is happy with his $210-a-month salary. The high point of his career, he said, came in 1986 when the Mumbai municipal commissioner, having heard of Mr. Harda’s prowess, came to see his work.

He brings to that work an exactitude that is ordinarily asked only of those who execute humans. Back in his office, he pulled out logbooks that he has kept since 1989. They list every rat catcher employed by B Ward and the tally of rats killed each month and year. Mr. Harda has often commissioned an artist friend to decorate the annual summary page with colored markers.

He is not alone in his devotion. Rat catching is one of those jobs that swallow you whole, said the top pest-control officer in Mumbai, Deepak R. Adsul, who is Mr. Harda’s boss. Even his antimosquito squad can never leave the office at the office, Mr. Adsul said. “When they’re having a stroll with their fiancées, they will look on the side of the road to see if larvae are there,” he said.

Mr. Adsul reached for the right analogy to explain the battle.

First he compared it to a chess game, then to the rivalry between India and Pakistan. The art is to know the enemy.

“You can be successful in this work only if you can imagine yourself in the shoes of a rat,” he said. “This is a war.”