This study investigated the age and growth, reproductive and feeding biology of the North American catfish Ictalurus nebulosus Le Sueur in the Waikato Valley of New Zealand, the changes that have occurred during acclimatisation and the extent of the differences in biology between here and other regions of the world. Under Waikato conditions the growth rate of the catfish was much faster than is usual for the species, although the longevity of about 8 years was the same as in the northern hemisphere. Oocyte development in the ovaries was asynchronous (Marza, 1938), with multiple spawning occurring. The duration of the spawning season was nine months, from winter to early autumn. Fecundity estimates were of a minimum of 3700 eggs laid per season. The major foods of the catfish were chironomid larvae, oligochaete worms, molluscs and crustacea. There was no evidence of piscivorous or fish-egg eating habits as has been noted for the species overseas. The catfish is expanding in numbers and range throughout the Waikato region because the recruitment capabilities of the population are high, with a rapid growth rate and high, fecundity, and there is an apparent lack of natural predators.