Fin-clip samples of African catfish broodstock were collected from four farms in Thailand (Table 1). We selected farms that had been operating for more than 10 years and where the broodstock in each farm had been separately managed. The individual samples were kept in 99% ethanol and delivered to the Fish Genetics Laboratory, Department of Aquaculture, Faculty of Fisheries, Kasetsart University, Bangkok. DNA extraction was performed using the standard phenol chloroform method (Taggart et al., 1992) with slight modification. The DNA was stored in TE buffer at 4 °C.