Biomethanation of banana peel and pineapple wastes studied at various HRTs showed a higher rate of gas production at lower retention time. The lowest possible HRT for banana peel was 25 days, resulting in a maximum rate of gas production of 0.76 vol/vol/day with 36% substrate utilization, while pineapple-processing waste digesters could be operated at 10 days HRT with a maximum rate of gas production of 0.93 vol/vol/day and 58% substrate utilization For pineapple-processing waste lowering of retention time did not affect the methane content significantly; however with banana peel an HRT below 25 days showed a drastic reduction in methane content. Copyright (C) 1996 Elsevier Science Ltd.