Fenitrothion organophosphorous insecticide was dusted on pota­to tubers which were then kept in the laboratory. Samples were taken for residue analysis 2 hours after treatment and then after 1, 2 and 4 weeks in order to study the persistence of fenitrothion. This study also included the removal effect of some processing steps (i.e. washing, peeling, blanching, cooking and drying) on the residues of fenitrothion. It was found that all of these steps caused significant decreases in feni­trothion residues. Another part of the study included a survey carried out for moni­toring the contamination of potato tubers from some local markets with organophosphorous insectisides. It was found that all organophosphor­ous residues detected exceeded the MRLs of codex except in one sam­ple for dimethoate insecticide which was below its MRL on potato tu­bers.