After weaning, groups of rats were fed a semi-purified diet containing fiddlehead greens (the unfuried frond of the ostrich fern Matteuccia struthiopteris), mature bracken fern (Pteridium aquilinum), or cycad meal (Cycas circinalis), or this diet plus aflatoxin B1. The groups fed 1-10% fiddlehead greens for 70-73 weeks developed no tumors, whereas those fed mature bracken fern for 67 weeks developed small-bowel adenocarcinomas (3/17). Those fed 3% cycad meal for 64 weeks developed tumors of the liver (4/18), kidney (2/18), small bowel (1/18), and large bowel (6/18). Those given aflatoxin B1 and observed for 73 weeks developed liver tumors (11/15). Thus this study indicates that fiddlehead greens from M. struthiopteris are not toxic or carcinogenic.