The flora and fauna show strong affinities with the Congo basin forest with many species reaching the eastern limit of their ranges in Semuliki Forest. The flora is dominated by a single tree species, Cynometra alexandri, mixed with tree species of a more evergreen nature. Swamp forest communities are also found.
The fauna of the forest is outstandingly rich and includes more than 400 bird species of which 216 (66% of the country’s total) are true forest birds such as the rare Sassi’s Olive Greenbul (Phyllastrephus lorenzi) and Forest Ground Thrush (Turdus oberlaenderi). Nine species of hornbills have been recorded as well. 75% of the Charaxes butterfly genuses are found in this forest, 441 species of bird, one species of primate, and one of butterfly are only recorded from this area in the East African part of their ranges. The forest is home to 53 mammals of which 27 are large mammals. 11 species are endemic to the park including the pygmy antelope and two flying squirrel species. It is also home to the peculiar water chevrotain, known as the “fanged deer”
Mammals include leopards, hippos, elephant, forest buffalo, hippopotamus, civets, potto, bush babies, mona monkeys, water chevrotains, and nine species of Duikers, including the Bay Duiker (Cephalophus dorsalis) and the Pygmy Flying Squirrel (Idiuus zenkeri) that occur nowhere else in East Africa.