The diet of the large pelagic fish, the southern opah Lampris immaculatus was examined along the Patagonian Shelf in the Falkland Islands region. Stomachs were available for 69 fish collected in 1993 and 1994. Surprisingly, this fish had a relatively narrow range of prey items. The single most frequent prey item was the onychoteuthid squid Moroteuthis ingens (predominantly juveniles) which was eaten by 93% of the fish. The other important prey were the loliginid squid Loligo gahi, the myctophid fish Gymnoscopelus nicholsi and the southern blue whiting Micromesistius australis. There was no evidence of larger individuals of L. immaculatus ingesting larger individuals of any of the 4 main prey species. An unexpected finding was the relatively high incidence of plastic ingestion (14% of fish). The plastic came from a variety of sources including food, napkin and cigarette wrappers and various pieces of plastic line and straps used in securing boxes. In several instances, there was evidence of feeding on fishing boat discards. The findings reveal a significant impact of plastic pollution in this region of the Southwest Atlantic.