The size and activity of atrazine-degrading populations are among the factors controlling atrazine persistence in soil. Populations of atrazine-degrading microorganisms in surface and subsurface soils were enumerated by the 14C-most-probable-number (14C-MPN) technique using [14C-ethyl]-atrazine or [U-14C-ring]-atrazine in C- or N-limited media. The [14C-ethyl] atrazine served as a potential carbon source for a larger number of microorganisms than [U-14C-ring]-atrazine. Populations of atrazine-degrading microorganisms using [14C-ethyl]-atrazine as a carbon source ranged from 4670 to 31,930 cells g-1 of surface soil, but were 1050 cells g-1 in a subsurface sediment. The size of these [14C-ethyl]-atrazine-degrading microbial populations were positively correlated (r2=0.81) to mineralization of [14C-ethyl]-atrazine added to soil, which suggests that atrazine-degrading populations may serve as an indicator of atrazine persistence in the soil environment. Populations degrading [U-14C-ring]-atrazine ranged from 130 to 1630 cells g-1 soil in C-limited media and from 19 to 10,530 cells g-1 soil in N-limited media. Population size and mineralization activity were generally increased by the frequency of atrazine use, suggesting that long-term exposure encourages microbial adaptation and growth.