Acute toxicity, morphological alterations and histological effects of an organophosphorus insecticide, profenofos (PFF) to earthworm, Eisenia foetida were evaluated by direct contact through a filter paper. The 24 and 48 h LC(50) was 4.56 and 3.55 microg cm(-2), respectively. Morphological and histological observations showed body ruptures, bloody lesions, and internal excessive formation of glandular cell mass and disintegration of circular and longitudinal muscles, which failed to regulate the internal coelomic pressure, leading to fragmentation in earthworms. Neurotoxic potentiality of PFF was assessed by measuring acetylcholinesterase (AChE). AChE basal K(m) and V(max) values were 0.26 mM and 0.15 micromol min(-1) mg(-1) protein, respectively. PFF alters the in vitro K(m) value, resulting in a competitive type of inhibition. The inhibitory constant K(i) was 2.52x10(-6) M. Morphological alterations and histological effects should be monitored in addition to AChE inhibition to assess OP pesticide contamination in soil ecosystems.