Meiosis is the developmental program by which diploid organisms produce haploid gametes capable of sexual reproduction. Here we describe the yeast gene AMA1, a new member of the Cdc20 protein family that regulates the multisubunit ubiquitin ligase termed the anaphase promoting complex/cyclosome (APC/C). AMA1 is developmentally regulated in that its transcription and splicing occur only in meiotic cells. The meiosis-specific processing of AMA1 mRNA depends on the previously described MER1 splicing factor. Several results indicate that Ama1p is required for APC/C function during meiosis. First, coimmunoprecipitation assays indicate that Ama1p associates with the APC/C in vivo. Second, Ama1p is required for the degradation of the B-type cyclin Clb1p, an APC/C substrate in both meiotic and mitotic cells. Third, ectopic overexpression of AMA1 is able to stimulate ubiquitination of Clb1p in vitro and degradation of Clb1p in vivo. Mutants lacking AMA1 revealed that it is required for the first meiotic division but not the mitotic-like meiosis II. In addition, ama1 mutants are defective for both spore wall assembly and the expression of late meiotic genes. In conclusion, this study indicates that Ama1p directs a meiotic APC/C that functions solely outside mitotic cell division. The requirement of Ama1p only for meiosis I and spore morphogenesis suggests a function for APC/C(Ama1) specifically adapted to germ cell development.