In this study, a nereistoxin analogue insecticide, thiocyclam, was administered to adult male albino rats by gavage dose of 135, 270 and 540. mg/kg b.w. repeated for 5. days at 24. h intervals. Control animals received only water. Thiocyclam was tested for its potential to cause genotoxic effects in rat bone marrow cells using an in vivo micronucleus assay. After 24. h of the last treatment, rats from all dose levels were sacrificed. Bone marrow cells were collected and assayed for the presence of micronuclei. Thiocyclam did not cause any increase in the incidence of micronucleated polychromatic erythrocytes in rats bone marrow at any of the dose levels. The polychromatic erythrocytes/normochromatic erythrocytes (PCE:NCE) ratio was found to be in the range from 0.50 ± 0.11 to 0.55 ± 0.02. The results of this study demonstrate that the effect of thiocyclam is not significant in the rat in vivo micronucleus assay. © 2010 King Saud University.