Fire Chief Dick Price says emergency personnel using oxygen and wearing protective gear are searching for the missing man in the sewer in Fairview, about 30 miles north of Dallas.
Price says the chances of the second worker being found alive are "pretty slim."
Fairview spokesman John Godwin says one worker was apparently overcome with fumes Thursday morning and another tried to save him, while a third sought help.
Price says the workers apparently were not wearing respiratory devices.
The men were contract workers for the North Texas Municipal Water District. Their names have not yet been released.