Samples of 11 brands of Swiss cheese were analyzed by plate count for molds capable of growing at 5 and 21°C. Portions of each sample were stored at 5°C and observed for visible mold development throughout a 6-wk storage period. Representative molds were isolated from counted plates and moldy samples, and identified to genus. Isolates were grown in yeast extract sucrose broth and rice powder corn steep agar cultures at 12°C for 2 wk, and then extracted with chloroform. The extracts were tested for toxicity to 7-day-old chicken embryos, and analyzed for the presence of known mycotoxins using TLC. Cheese samples that developed visible mold growth during storage were extracted with acetonitrile. The extracts were examined for the presence of known mycotoxins using TLC. Mold counts ranged < 10 to 1580 colonies/g of surface cheese at 5°C and from < 10 to 5700 colonies/g of surface cheese at 21°C. AlI cheese samules stored at 5°C develoued visible mold growth within 6 wk. Of the 183 molds isolated, 87% were Penicillium species; 93% of the isolates that grew at 5°C were penicillia. Toxicological screening of the mold isolates showed extracts of 34% of all isorates, and 35% of Penicillium isolates were toxic to chicken embryos. Chemical analyses of the culture extracts detected known mycotoxins in 5.5%. Toxins detected were penicillic acid, patulin and aflatoxins. Analyses of moldy cheese stored at 5°C for 6 wk for known mycotoxins showed peniciliic acid in 4 of 33 samples. The isolated penicillic acid was comparable to standard penicillic acid by TLC in three different solvent systems using three different derivatization procedures. UV and IR absorption spectral data supported the TLC data.