The species richness of flora and fauna in the Nzoia River drainage basin is documented through a study of museum specimens, catalogues and databases. The catchment area and basin covers 2.2% (12900/580367 km²) of Kenya's total land area with an altitudinal range of 1140 to 4300 m and varied ecosystem and land uses. We recorded approximately 9.3% (3239/34677) of Kenya's current known species of vascular plants, invertebrates (insects and spiders), fish, amphibians, reptiles, birds and mammals. Bird species made up the highest proportion 58.3% (650/1114) of the national total followed by amphibians 37.3% (41/110), reptiles 45.0% (86/191), mammals 31.3% (122/390), vascular plants 17.9% (1251/7000), fish 6.7% (58/872) (32.2% (58/180) for freshwater fish only) and invertebrates (insects and spiders) 4.1% (1031/25000). Ninety-five species recorded in this area are endemic to Kenya and 42 globally threatened. The species recorded contribute to several ecosystem services including pest control, pollination, bio-indicators, medicine and cosmetics, building materials, ecotourism, research and education. Data available differed substantially across counties and taxon groups with gaps apparent in five counties (Bungoma, Busia, Elgeyo Marakwet, Siaya and Usain Gishu) and four taxa plants, invertebrates, fungi and bacteria where a dearth of information exists. To fill these gaps we recommend prioritisng future survey effort on taxa and counties with fewer than 10% of the total numbers of records.

A total of 605 vascular plant species, including flowering plants, gymnosperms and ferns, are included in this annotated checklist for Arabuko-Sokoke Forest Reserve in Kilifi County in the Coast Region of Kenya. Out of these 491 are supported by a voucher specimen in the East African herbarium, and 106 are sight records, whereas an additional 8 are cited from the Flora of Tropical East Africa (FTEA). This total represents approximately 9% and 18% respectively of the flora of Kenya and the coastal region (K7 of FTEA). Taxa are distributed in 103 families and 389 genera, the largest families being Leguminosae (Fabaceae) with 62 taxa, Rubiaceae with 53 and Malvaceae with 26. The largest genera include Grewia, Cyperus, Ficus and Ipomoea. For each record a name, a cited specimen or sight record, a short general description, and its known phytogeographic distribution is provided. Globally threatened species are also indicated. The conservation importance of Arabuko-Sokoke is discussed.