A case of acute diarrhoea caused by Aeromonas trota (formerly HG 13 group) in a Spanish child is reported. The strain was isolated in the faeces using the CIN agar (cefsulodin-irgasan-novobiocin) culture media. The strain was initially identified as A sobria by the commercial GNI card and API 20E biochemical systems. The strain was, however, VogesProskauer and sucrose negative, so complementary tests of cellobiose fermentation and gluconate oxidation were performed. These tests, together with the strain susceptibility to ampicillin (MIC 1 microgram/ml) and carbenicillin (MIC < 16 micrograms/ml) led to the final identification of A trota. The microbiological characteristics of this new species and the principal tests required for its identification are presented. The isolation, for the first time, of A trota in the Mediterranean area confirms the suspected worldwide distribution of this species.