In South Africa’s Western Cape, the main breeding season of Hartlaub’s Gull (Larus hartlaubii) is from January or February until May. At southern localities, there may be a second peak in breeding in August and September. Primary molt of adults mostly starts between August and December and lasts on average 115 days. In each of nine body measurements, averages were higher for males than for females, but values overlapped. In Western Cape during 1990-2001, the species bred at nine localities where breeding had not previously been reported. The maximum number recorded breeding at all localities in this region in any one year was about 5,500 pairs in 1995, less than the 10,000-11,000 pairs estimated for the region in the 1980s. Because the timing of the peak in breeding varies from year to year, it is considered that the 5,500 pairs represent an under-estimate, but it is likely that numbers did decrease after the 1980s. There are substantial inter-annual fluctuations in numbers of birds breeding, suggesting that in some years an appreciable proportion of the adults do not breed.