Extra-pair copulation (EPC) and within-pair copulation (WPC) behaviour of red-billed gulls (Larus novaehollandiae scopulinus) was observed in a breeding colony at Kaikoura Peninsula, New Zealand. Twenty-five females and 19 individually colour-marked males were monitored for six hours per day for twenty days from 20 September to 30 October 1973. Extra-pair copulation attempts were common, amounting to 21% of all copulation attempts, but less than 3% were successful during the fertile period of the female. Ten percent of EPCs were on males. Within-pair mountings were eight times more likely to end in cloacal contact than EPCs. Within-pair and EPC attempts occurred throughout the day, but the frequency of WPC attempts increased markedly towards evening, possibly as an adaptation to ensure last-male sperm precedence. Approximately 80% of females, but only 32% of males, were involved in EPC attempts. Mounting within and between pairs increased in frequency about 15 days prior to laying of the first cgg. Within-pair copulation attempts ceased abruptly after the first egg was laid, but EPC attempts on the female continued well into incubation and amounted to 11% of the total EPC attempts. Seventy-five percent of EPC attempts occurred in the fertile period of the recipient female. Approximately 75% of the EPC attempts by males occurred over a nine-day period; six days prior to and two days after its mate had laid the first egg. No EPC attempts were recorded for males more than seven days after its mate commenced laying. The female had control as to whether the copulation attempt would be successful. In WPC attempts the female ended the mount on 72% of the occasions, whereas all EPC attempts were ended by the female. Approximately 91% of within-pair mountings followed some form of courtship feeding display, and in the remaining 9% the male mounted without courtship preliminaries. In contrast, 97% of extra-pair mountings occurred in the absence of courtship displays. On the two instances where preliminaries occurred, mounting followed courtship feeding solicitation by the female towards the strange male. Potentially high quality females which were being well provisioned in courtship feeding by their mates were at greater risk from EPCs because they were able to spend more time at the nest site than less well provisioned females. Females which were well fed during courtship feeding resisted all EPC attempts and retained their partner the next breeding season. Poorly courtship fed females divorced the next season. One such female solicited an EPC four days prior to the laying of her first egg. There was no evidence to suggest that males performing EPCs were at risk from being cuckolded and the male partners of females experiencing high numbers of EPCs did not respond to the risk by having more WPCs or having more genital contacts per hour. It is considered that EPCs were not a major feature of the mating system for the majority of red-billed gulls. The advantages of EPCs were greater for females than for males and the results support the genetic quality hypothesis. Theoretically if males wanted to maximise their fitness they should attempt EPC's on females nesting earlier than themselves, but this only happened on 17% of the EPC attempts. The high number of WPCs, the increase in frequency of copulations in the evening and high courtship feeding rates are measures that help to ensure paternity of the true mate. It is hypothesized that in species like the red-billed gull which have long-term pair-bonds and invest considerable time and energy in courtship feeding and parental care during incubation and chick rearing it would be more advantageous to strengthen the pair-bond than to philander to increase production. In a mating system such as this, philandering would jeopardize the current reproductive investment and future reproduction because those which change partners are less productive than those which retain their partners and for those which divorce the probability of breeding the next season is lower than for those which retain pair-bonds (MILLS, unpub. data). More successful breeding occurs if the pair-bond has been established for more than one year (MILLS, unpub. data). Thus, in this mating system the ''attentive prosper''.