Heterozygosity in three cultivars of blackcurrant (Ribes nigrum) was estimated. A selfed population of each cultivar was screened for RAPD markers and heterozygous loci were identified by band segregation in contrast with the non-segregation of homozygous loci. On average, 21% of the loci scored in each cultivar were heterozygous. The implications for mapping studies in R. nigrum are discussed.