A new species, Acaenodera spinosior, is described based on specimens collected from the intestine of Conger orbignyanus Valenciennes (Pisces: Congridae) caught in waters off Mar del Plata (38 degrees 08'S, 57 degrees 32'W), Argentina. Five of 36 congers examined were parasitized (prevalence: 13.9%, mean intensity: 2.6+/-1.52, range: 1-5). The new species is distinguished from its only congener, Acaenodera placophora Manter and Pritchard, 1960, a parasite of congers off Hawaii, by its larger size, lacking eyespots, and exhibiting a greater number of circumoral, mid-ventral and lateral spines. This is the first record of Acaenodera Manter and Pritchard, 1960 in Atlantic waters.