Monitoring populations of pests and their natural enemies under different management situations and seasonal weather parameters provides extremely useful information for taking preventive measures against pest outbreaks. The abundance, spatial and temporal distributions of herbivorous insects and arthropod natural enemies on Alnus species were monitored at four sites in Kabale district, Uganda between June 1999 and August 2000. Chewing insects, dominated by Coleoptera (75%), constituted the majority of insect herbivores sampled. Dominant and potentially serious pests of Alnus included Apion globulipenne, an unidentified Chrysomelidae (Coleopt. 27), Phymateus viridipes, Coloborrtics corticina and a Cacopsylla species (Homoptera: Psyllidae). Spiders were the predominant natural enemies accounting for 64% of the total natural enemies encountered, followed by parasitic Hymenoptera (30%). There were marked spatial and temporal variations in arthropod abundance. Among sites, mean abundance of total insect herbivores and total natural enemies on A. acuminata over 15 months ranged from 3.8-8.5 and 3.3-4.7 individuals per 1-m branch length respectively. Over the same period, mean number of total insect herbivores and total natural enemies on A. nepalensis that was studied at only one site were 11.9 herbivores and 4.2 natural enemies per 1-m branch length. Populations of most insect orders increased in the wet season although the greatest herbivore abundance was evident in the dry season. Further studies are necessary on the impact and management strategies of the potentially important insect pests and natural enemies on Alnus.