The compatibility system in Streptomyces fradiae was investigated. When 108 conidia of each of 2 freshly isolated auxotrophic mutants are mated on minimal medium, usually 1 to 10 primary heterokaryotic colonies appear (low-frequency mating, LFM). When secondary parental isolates, derived from the conidia produced by primary heterokaryons, are mated the frequency of heterokaryons is increased several thousand-fold. This high-frequency mating (HFM) is a reasonably stable property. A mutational origin is therefore postulated for the HFM and LFM isolates. More effective anastomosis, rather than increase in residual growth appears to be responsible for HFM. Interspecific crosses were unsuccessful, even when an HFM tester stock was used; heterokaryotic interaction between independently isolated auxotrophic mutants derived from the same line of S. fradiae seemed to increase when an HFM strain was one of the partners.