Sexual development of Dictyostelium discoideum is a unique and useful system for the study of sexual phenomena. We have been studying molecular mechanisms of sexual cell fusion in D. discoideum and have identified several relevant cell-surface proteins. One of the proteins, gp138, was identified as a target molecule for fusion-blocking antibodies, and two genes for gp138, GP138A and GP138B, were cloned. The participation of gp138 in the sexual cell fusion was confirmed by antisense RNA mutagenesis, but it is unclear which of the genes encodes gp138. Moreover, the presence of a third gene for gp138 was indicated by gene disruption. In the present study, we generated strains of D. discoideum overexpressing either GP138A or GP138B to investigate the products of these genes. The transformants overexpressing GP138A and GP138B overproduced glycoproteins with molecular masses of 135 and 130 kDa, respectively. Although their molecular masses were different from that of gp138, the results of peptide mapping and amino acid sequencing showed that they are related to proteins, suggesting that the proteins encoded by GP138A and GP138B are isoforms of gp138 protein.