Meal size and feeding frequency of Christmas shearwaters [Puffinus nativitatis] and Phoenix petrels [Pterodroma alba] on Motu Tabu, Christmas Island, Central Pacific Ocean, were determined by weighing chicks at 4 h intervals throughout 24 h periods. Most feeding occurred between 1600 and 2400 h. Meal size was estimated by the mass increment between visits. Positive increments were summed over 24 h (SUM) to provide an index to the total mass of food received. Over 2 nights, 10 of 17 Phoenix petrel chicks had SUM exceeding 25 g (max, 110 g), indicating large meals. Four SUM of 5 and 10 g were discounted as either weighing errors or inconsequential meals. Assuming that members of a pair fed their chicks independently, this indicates a probability of feeding of 0.36/parent per night, i.e, every 3rd night on average. Of 30 Christmas shearwater chicks, 26 had SUM exceeding 35 g (maximum 95 g), suggesting a feeding probability of 0.63/parent per night. The means Of SUM were 54.8 .+-. 23.5 SD for the Christmas shearwater (16.1% of chick mass) and 37.1 .+-. 36.8 SD for the Phoenix petrel (13.6%). Estimated masses of meals delivered by each parent were 48.2 g .+-. 9.1 SD (14.2% adult mass) and 49.4 g .+-. 21.5 SD (18.2%), respectively. Net mass gains by chicks over 24 h were significantly related to SUM with slopes of 0.42 g g-1 .+-. 0.07 SE and 0.77 g g-1 .+-. 0.11 SE, respectively. Data are consistent with the suggestion of Ashmole and Ashmole (1967) that the Phoenix petrel feeds more pelagically than the Chrismas shearwater, and delivers a meal with a higher energy content and less water. Mass increments can be analyzed to describe components of variation in meal size and feeding rate for comparison among pelagic seabirds. Data may also allow one to infer aspects of feeding ecology and the resource environment of the chick.