Plankton samples for rotifers were collected by means of vertical hauls with phytoplankton nets at eight stations on Lake Albert during the course of a year. Similar hauls were taken from Lakes Kyoga, Victoria, George and Edward in October and November. The rotifers found in the samples are listed and some estimates of their abundance and seasonal occurrence in Lake Albert are given. The most important planktonic rotifers are Keratella tropica and several species of Brachionus. Other species may also become locally or temporarily abundant; Lecane bulla becomes numerous in samples taken near vegetation after disturbance by rough weather, or where blue-green algae are abundant.
The associations of rotifers at the stations on Lake Albert and in the other lakes have been compared by means of the Sorensen Index and the index of diversity. The highest diversity is found in situations with a high rate of flow, as at the mouth of the Victoria Nile, where the extra species are non-planktonic forms swept into suspension by tile current. This high diversity is associated with low numbers of individuals per unit volume. In Lake Albert rotifers are most consistently present and abundant at the mouth of the River Semliki, where the rate of flow is moderated by a large reed swamp. The middle of Lake Albert is poor in rotifers, and this may be related to the sparseness of planktivorous fish coupled with competitive elimination of the rotifers by larger crustacean zooplankters.
The associations of rotifers in Lakes Kyoga and George are similar, and resemble one another more than they resemble the associations in their neighbouring deeper lakes. The association in Lake Kyoga also resembles the associations found in water of a similar depth at the northern end of Lake Albert.