A clinical isolate of Proteus mirabilis (Pr 990) and an isogenic non-flagellate allelic replacement mutant (Pr M9) were tested for their ability to cause infection in the ascending mouse model of urinary tract infection. Wild-type Pr 990 differentiates into swarmer cells in brain-heart infusion broth. Pr M9 neither has flagella nor does it apparently differentiate into swarmer cells after subculturing. The infectivity of both strains from an initial culture and the sixth subculture was assessed in the ascending urinary tract infection mouse model. Infection was ascertained by determining colony forming units from kidney and bladder homogenates from individual mice 7 days after inoculation. In all cases the nonflagellate mutant Pr M9 was at least as infective as Pr 990. Using bacteria from the first culture, Pr M9 infected 61.5% and Pr 990 infected 45.5% of mice tested. The levels of viable counts were similar between the Pr M9 and the Pr 990 infections. Using bacteria from the sixth subculture, Pr M9 infected 75% and Pr 990 infected 76.5% of mice tested. Again viable counts were similar. Pr 990 increased in infectivity from the first to the sixth subculture, whereas Pr M9 did not, but this may be a reflection of the high initial rate of infectivity with first culture Pr M9. These results suggest that neither flagella nor swarmer cells are required for P. mirabilis infectivity in ascending urinary tract infections in mice.