An increased reactive oxygen species (ROS) and insufficient antioxidant activity is known in diabetes mellitus (DM). Antioxidant compounds in the human foods or supplementary diets can be used to counteract several diseases. The analysis of micronuclei (MN) is a cytogenetic technique used to show chromosomal damage caused by clastogenic affects. The present study was designed to evaluate: (i) the effects of diabetes mellitus on bone marrow MN frequency, (ii) the effect of oral administration of Ulva rigida ethanolic extract (URE) on MN frequency produced by DM, and (iii) some hematological values in normal and streptozotocin-induced diabetic rats. Daily fluid and food consumptions, weekly body weights, blood glucose concentrations and serum insulin levels were also examined in the study groups during the two different administration periods. The blood glucose concentration and MN frequency have been significantly increased in diabetic rats compared with the normal rats (p < 0.0001). Especially, URE-30d group treatment in diabetic rats was significantly decreased blood glucose concentrations and MN frequency. This is the first report on the anti-hyperglycemic, anti-oxidative and genotoxic/antigenotoxic capacity of U. rigida in vivo. Our results suggest that URE shows strong anti-hyperglycemic and antigenotoxic effect on the genotoxicity produced by DM in rats. (C) 2009 Elsevier Ltd. All rights reserved.