Crude cell extracts of Neurospora crassa contained an abundant protein that was identified as actin by a number of criteria. The protein, either in cell extracts or in pure form, co-migrated with rabbit skeletal muscle actin in polyacrylamide gels. The N. crassa actin was purified by DEAE-cellulose and DNAase I-Sepharose chromatography and had the expected property of inhibiting DNAase I activity. Although N. crassa actin could polymerize and depolymerize, purification based entirely on this characteristic was ineffective. The actin was susceptible to proteolytic degradation, and under certain conditions, a breakdown product of defined size was observed.