Tests were conducted to determine the effects of leptophos, phorate, terbufos, and WL 24073 {0‐ [2‐chloro‐1‐(2,5‐dichloro‐phenyl) vinyl]0‐methylethylphosphonothioate} on microbial populations and activities in a sandy loam. A stimulatory effect on bacterial numbers was observed with some insecticide treatments in the early stages of incubation. WL 24073 increased the nitrification of the incorporated (NH4)2SO4‐N. Phorate stimulated soil microbial respiration.