Abstract Ribosomes from Blastocladiella emersonii zoospores stimulate in vitro protein synthesis in a system using soluble factors extracted from wheat germ. Aurintricar☐ylic acid inhibits less than 40% of the in vitro protein synthetic activity of the zoospore ribosomes, indicating that messenger RNA is already complexed to the ribosomes. In addition to the mRNA complexed to the ribosomes, zoospores contain an mRNA fraction which is not bound to the ribosomes. Extraction of RNA from zoospore ribosomes and deacylation followed by reacylation with labeled amino acids demonstrated the presence of tRNA molecules specific for methionine and other amino acids on zoospore ribosomes. Transfer RNA from zoospore ribosomes contained 9.8% methionyl-tRNA compared to 2.4% methionyl-tRNA bound to ribosomes isolated from growth-phase plants. The fourfold enrichment of methionyl-tRNA on zoospore ribosomes suggests that between 12 and 25% of the zoospore ribosomes exist in arrested 80 S initiation complexes. Collectively, the data indicate that zoospore ribosomes complexed to mRNA have completed initiation and are somehow blocked at one or more of the elongation steps of protein synthesis. The data are compatible with the idea that an inhibitor is associated with the zoospore ribosomes in vivo.