Wastes from cassava processing mills are usually discharged uncontrollably into the environment where they cause serious environmental challenges. This research work was undertaken to investigate the effects of cassava mill effluent on the microbiological quality of soils and to determine the effects of the polluted soils on the growth of plants. Fresh cassava mill effluent and unpolluted soil samples were collected and analyzed microbiologically. The soil sample was polluted with the cassava mill effluent and the polluted soil was also subjected to microbiological analyses for 31days. Germination and growth of pumpkin plants were also investigated using soil samples polluted with different concentrations of the cassava mill effluent. The microbial population of the effluent showed presence of Staphylococcus aureus, Bacillus spp, Streptococcus spp, Pseudomonas aeruginosa, Aspergillus spp, and Candida spp. The bacterial count was 1.4x104cfu/ml while the fungal count was 1.1x103cfu/ml. The results of the analyses of the unpolluted soil and the effluent polluted soil samples showed that the effluent had noticeable effects on the physicochemical and microbiological properties of the soil. The total microbial count increased from 2.2x105 cfu/g to 4.6x105cfu/g for bacteria and 1.6x104cfu/g to 3.3x105cfu/g for fungi. The polluted soil showed higher diversity of micro-organisms. The isolated organisms were: Staphylococcus aureus, Bacillus spp, Streptococcus spp, Pseudomonas aeruginosa, Micrococcus spp, Proteus spp, Enterobacter spp, Aspergillus spp, Candida spp, Penicillium spp, and Rhizopus spp. The polluted soil also, at higher concentrations of the effluent inhibited the germination of plants. This research revealed that effluents from cassava processing mills have significant effects on the microbiological properties of the surrounding soils and also affected the growth of plants on these soils negatively. It therefore suggested that this effluent should be treated properly before disposal to the environment.