In this study, the cytogenetic effects of Marshal (insecticide/nematocide) were investigated in bone marrow cells of rats. The results obtained from animals treated with Marshal were compared with the results of animals treated with ethyl carbamate (EC) and with controls. Concentrations of 12.5, 25 and 50 mg/kg b.wt. of Marshal and 100, 200 and 400 mg/kg b.wt, of EC were used and animals were sampled at three different times (6, 12 and 24 h). Marshal increased the number of chromosomal aberrations (CA) per cell, and the number of cells with abnormalities, at all concentrations and treatment times. Generally, Marshal could increase the number of the abnormal cells and the formation of CA as easily as EC. However, Marshal, at 50 mg/kg b.wt. did not increase the frequency of abnormal cells or CA as strongly as EC, at 400 mg/kg b.wt. for 6 h. Marshal also decreased the mitotic index (MI) compared with the control group. The MI was higher in the group treated with Marshal for 6 h than that treated with EC. However, the effects of Marshal and EC on the MI in the groups treated for 12 and 24 h were similar. We found that the effect of Marshal on the formation of abnormal cells and CA was dependent on concentration and treatment time.