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We studied the breeding biology of a colony of Caspian terns (Sterna caspia) near Invercargill, New Zealand, during 1992 and 1993. The mean clutch size did not differ between years and averaged 2.04. Measurements of 147 eggs averaged 64.5 x 44.6 mm; there was no difference in size of A-eggs (1st-laid in a clutch) and B-eggs (2nd-laid) in either year, but the few C-eggs laid were significantly smaller. The incubation period averaged 27.2 days (range 26-29 days); some earlier published values of 20-22 days appear to be in error. In 1992, growth rates of A-chicks were significantly higher than those of B-chicks. Growth rates of A-chicks were significantly higher in 1992 than in 1993. Fledging occurred at 33-39 days at an estimated average mass of 527 g in 1992 and 501 g in 1993. Minimum productivity was 1.04 and 0.62 chicks fledged per pair in 1992 and 1993, respectively. Weather during the period of chick growth was much wetter and windier in 1993 and we suggest that this reduced the ability of parents to feed chicks. Investigator disturbance, which has been implicated in lower reproductive success in some studies of Caspian terns, did not appear to have a major impact in our study. We believe this was partly because the birds were habituated to our activities and partly because of our methodology.