Efforts are being made in Tanzania to promote mushroom cultivation, and identification of abundantly available plant biomass residues appropriate for growing mushrooms is part of the efforts. This study investigated the suitability of water hyacinth as a bulk substrate for growing a newly domesticated local oyster mushroom, Pleurotus flabellatus. The performance of the mushroom was investigated under ambient temperature and relative humidity (RH) regimes of 18-25/27-29 oC and 55-85/78-93%, respectively. The growth cycle of the mushroom was completed in 40 days with three and four flushes respectively. At the higher temperature and RH regime, the mushroom grew faster and the first flush was harvested at the 13th day after substrate inoculation with a Biological Efficiency (B.E.) of 84%, whereas the first harvest was done on the 19th day after inoculation at the lower temperature and RH regime with a B.E. of 53%. Substrate total fibre loss at the end of the growth cycle was in the range of 31-40%, and cellulose the most utilized fraction, decreased by 35-48%. The rates of fibre loss increased over time during the mushroom growth and were highest during the first and second flush during which about 80% of the total mushroom yield were obtained. Water hyacinth shoots proved to be a good substrate for growing the local oyster mushroom at ambient environmental conditions.