The fission yeast Schizosaccharomyces pombe and the budding yeast Saccharomyces cerevisiae are highly diverged (530 mya), single-celled, and model eukaryotic organisms. Scientists employ mating, meiosis, and the plating of ascospores and cells to generate strains with novel genotypes and to discover biological processes. Our 3 laboratories encountered independently sudden-onset, major impediments to such research. Spore suspensions and vegetative cells no longer plated effectively on minimal media. By systematically analyzing multiple different media components from multiple different suppliers, we identified the source of the problem. Specific lots of agar were toxic. We report that this sporadic toxicity affects independently the agar stocks of multiple vendors, has occurred repeatedly over at least 3 decades, and extends to species in highly diverged taxa. Interestingly, the inhibitory effects displayed variable penetrance and were attenuated on rich media. Consequently, quality control checks that use only rich media can provide false assurances on the quality of the agar. Lastly, we describe likely sources of the toxicity and we provide specific guidance for quality control measures that should be applied by all vendors as preconditions for their sale of agar.