The prevalence and degree of antibiotic resistance in catfish and eel farms in the southern part of The Netherlands was examined using motile aeromonads as indicator bacteria. A total of 29 water samples were collected, originating from six catfish farms, one catfish hatchery and three eel farms, and were plated on an Aeromonas-selective agar with and without antibiotics. From each plate, one colony was screened for presumptive motile aeromonads and tested for antibiotic susceptibility. The prevalence of resistance was as follows: ampicillin and oxytetracycline 100%; sulfamethoxazole 24%; trimethoprim 3%; and ciprofloxacin and chloramphenicol 0%. The majority of samples showed a high degree of oxytetracycline resistance, implicating fish farms as a major reservoir of oxytetracycline resistance genes. This reservoir might form a risk for human health and has major consequences for the effectiveness of this antibiotic in the treatment of infectious diseases in fish.