Where the Avon and Heathcote rivers meet before discharging into the sea, an area of mudflats has been formed, commonly termed the Avon – Heathcote or Christchurch estuary. It is roughly a triangular shaped region, approximately three square miles in area. Of this area about ¾ consists of exposed mudflats at low tide. Previous studies on the estuary biota have been those of Thompson (1929), Bruce (1953), Williams (1959). Rosenberg (1962), and Estcourt (1962). The last author has an excellent review of previous work. The aims of the investigation were as follows: (1) To provide a general survey of the fish population of the Christchurch estuary, as a basis for future comparison with other New Zealand estuaries. (2) To investigate the feeding, sexual maturity and size relationships of the estuarine fish populations, and relate these to the respective habitats occupied. (3) To note the occurrence, and where possible, the incidence of the fish parasites. (4) To provide data and material for future research work on estuarine fish biology. (5) To provide, where possible, a brief and simple account of relative growth rates. The present investigation was started on 23rd April. 1965, and owing to unforeseen circumstances, had to be terminated on the 8th December, 1965. However, various additional samples were taken at certain periods after this date (see appendix 1).