The breeding biologies of the black-crowned night-heron (Nycticorax nycticorax) and the little egret (Egretta garzetta) were studied in the mixed heron breeding colony at the Huleh Nature Reserve, Israel, from 1977 to 1980. Both studied species have resident (30%) and migrating populations. The colony consisted of about 3,000 pairs of seven breeding species nesting in 5-7 highly synchronous mixed species sub-colonies from mid-March to mid-September. Mean time-lapse between initiation of two sub-colonies is 20.45 days. Mean duration of egg-laying within a sub-colony is 12.45 days and of hatching is 12.38 days. Nests are built each year mainly with papyrus stems and there are significant differences among little egrets, night-herons and cattle egrets in their tendency to use different nesting material. Mean clutch size was 3.41 for little egrets and 2.97 for night-herons, while the most frequent clutch size was three in both species (54.9% and 68.5%, respectively). Mean fresh laid egg masses were 27.15 g and 31.33g for little egrets and night-herons, respectively. The variability in egg mass within a clutch was significantly smaller (on average 12 times and 7 times for little egrets and nightherons, respectively) than between clutches. Incubation of each egg lasts 21.6 days in little egrets and 21.1 days in night-herons. Mean egg mass loss during incubation was 14.6% and 13.8% for little egrets and night-herons, respectively. Hatching success is 90.2% (n= 177) in little egrets and 92.4% (n= 258) in night-herons, and hatching order is related to laying order in both species. Hatchling's mass at hatch ranged from 13-26 g (n= 34), 74.8% of initial egg mass for little egrets and 19-29 g (n= 30), 73.8% of initial egg mass for night-herons. Chick development rates were studied in nature and in captivity. In both species there is a linear correlation between a chick's age and its body dimensions. Chicks reach adult dimensions at about one month and fledge at the age of 35-42 days. Fledged chicks disperse during the first year up to 50-60km from the colony. Survival rates for chicks up to the age of 28 days are 69.9% and 56.6% for little egrets and night-herons, respectively. The main mortality factor for eggs and chicks is probably predation by avian predators. Data on minimum longevity, 10-11 years for little egrets and 7-8 years for night-herons, were obtained from resightings of tag-marked individuals. Long-term monitoring and study of the structure and breeding parameters of a regionally important colony in relation to known environmental effects are an essential tool for conservation and management plans for colonial species.