Three previously reported methanesulfonate insecticides, 6isobutylthio-2-pyridyl methanesulfonate (I) and its sulfoxide (11) and sulfone (111) analogues were examined in two insect species Lucilia cuprina and Blattella germanica and in tissues from vertebrates. The results of B. germanica tests and cholinesterase assays confirmed the insecticidal activity of the compounds, with cholinesterase inhibition being the most likely mode of insecticidal action. The inactivity of the sulfide I and sulfoxide I1 in vertebrate in-vitro studies may indicate that conversion, in vivo, of the sulfide and sulfoxide methanesulfonates to the sulfone (111) is a requirement for activity. In mouse toxicity tests, matching high toxicity was observed for the alkylthio-, alkylsulfoxy-and alkylsulfone analogues indicating fast metabolic oxidation of the injected alkylthio-and alkylsulfoxy-compounds. However, in in-vitro tissue tests, the sulfone, although active, did not exhibit the characteristic pharmacological profile of the standard acetylcholinesterase inhibitor, physostigmine. The sulfone demonstrated a mixed action, with indications that it acts as an inhibitor of specific cholinesterase isozymes, or that it may modify responses at cholinoceptors.