A semi-arid soil treated with different concentrations of formulated atrazine in a laboratory experiment was studied over 45 days, by different biological and molecular parameters (bacterial enumeration (cfu), community level physiological profiles (CLPPs) measured by Biolog® and denaturing gradient gel electrophoresis (DGGE)), to study the bacterial community diversity.Formulated atrazine was almost totally degraded at different concentrations after this incubation time. The number of colony forming units (cfu) for soils with 100 and 1000mgkg−1 atrazine was significantly (p≤0.05) higher than for the control, 1 and 10mgkg−1 treatments. DGGE banding patterns showed that regardless of time elapsed, concentrations of 10, 100 and 1000mgkg−1 atrazine in soil, affected the bacterial community compared to control and 1mgkg−1.The Shannon diversity index (H′) based on CLPP data showed a significant (p≤0.05) decrease at atrazine concentrations of 100 and 1000mgkg−1. The Shannon diversity indices for different guilds of source carbon and the parameters K and r (based on the kinetics of colour formation rather than on the degree of colour development) were related to guilds of carbon substrates and atrazine concentration at a sampling time. The parameter K was very sensitive to atrazine effects on microbial communities.These biological and molecular parameters can be used to monitor changes in soils treated with atrazine at different concentrations, even when the pesticide is degraded.