Little is known about the impacts of persistent triazine herbicides and biocides on soil microorganisms. Terbutryn toxicity in soil microorganisms was studied using bacterial and fungal growth, substrate induced respiration (SIR) and basal respiration as ecotoxicological end-points. In the short-term (0–7 days), increasing concentrations of terbutryn (0–800 mg kg⁻¹) progressively inhibited bacterial and fungal growth by up to 33–36% (4 h) and 49–55% (7 days), whereas SIR and basal soil respiration remained unaffected. Following long-term (40 days) exposure to terbutryn, both bacterial and fungal growth were inhibited by up to 76–78%, and SIR was inhibited by up to 53%. Hence, our results unexpectedly demonstrate time-cumulative microbial growth inhibition over extended time periods in soil and indicate that current ecotoxicological guidelines may underestimate risks posed by chemicals to soil microorganisms.