To accelerate the breeding of Agaricus bisporus, quick and reliable methods to identify the infrequent homokaryons are necessary. A new marker, inter simple sequence repeat (ISSR) fingerprinting, is described for differentiation of homo- and hetero-karyotic protoclones. Nine slow growing protoclones, two strandy and seven appressed, were analyzed for the first time with ISSR amplifications. The patterns were highly polymorphic and very reproducible. Among 40 primers tested, 7 ISSR primers were selected for the analysis of genomic DNA and generated a total of 68 ISSR fragments. ISSR fingerprinting detected 44.12% polymorphic loci. All appressed homokaryons carried a subset of ISSR markers found in the heterokaryons, and clustered separately in dendrogram. These were not able to produce a fruiting body. A test of cross-fertility and the following fruiting trial proved that 7 of the 9 protoclones with different ISSR fingerprints were homokaryons. These results demonstrated that ISSR markers provide an efficient alternate for identification of homokaryons and suggest these markers be considered as new tools for the survey of Agaricus species.