In the present study, we have evaluated the clastogenic and/or aneugenic activities of the lyophilized extract of the rhizome of Cochlospermum regium Mart. (algodãozinho-do-campo), which has been used by Brazilian population as anti-inflammatory, analgesic, anti-bacterial and anti-edematogenic agents. The mouse bone marrow micronucleus test was applied in erythrocytes of mice. Groups of 5 animals were treated with lyophilized extract of "algodãozinho-do-campo" by intraperitoneal administration (i.p.) in doses of 19 mg/kg (10% of LD₅₀), 38 mg/kg (20% of LD₅₀), 76 mg/kg (40% of LD₅₀) and 114 mg/kg (60% of LD₅₀). For all doses, the micronucleated polychromatic erythrocytes frequency (MNPCE) was evaluated 24 h after treatment. The cytotoxicity was evaluated by polychromatic and normochromatic erythrocytes ratio (PCE/NCE). The results showed that the extract of C. regium in dose of 19 mg/kg (10% of LD₅₀), caused no significant increase at the frequency of MNPCE in relation to negative control group (p>0,1). However, in doses of 38 mg/kg (20% of LD₅₀), 76 mg/kg (40% of LD₅₀) and 114 mg/ kg (60% of LD₅₀), this extract provoked significant increase of MNPCE frequency (p<0,05) when compared to negative control. Thus, we can suggest that the lyophilized extract of rhizome of Cochlospermum regium Mart. exhibited the mutagenic activity. The cytotoxicity was shown in doses of 38 mg/kg (20% of LD₅₀), 76 mg/kg (40% of LD₅₀) and 114 mg/kg (60% of LD₅₀).