Soil fertility decline and fallow effects were studied in Ferralsol-Acrisol catenas of plantations of sisal (Agave sisalana) in north-east Tanzania. The fertility of Ferralsols that had been subject to continuous sisal cultivation in the absence of fertilizers was extremely low but that of Ferralsols that had been under 18 years of bush fallow or under secondary forest was slightly better. Acrisols that had been under continuous sisal cultivation were less depleted than the Ferralsols because of greater intrinsic fertility. A comparison of soil analytical data from the 1950s and 1960s with recent data from the same sisal fields showed that the topsoil pH of the Ferralsols had decreased by 1.5 (r2 = 0.807) and that of the Acrisols by 1.2 (r2 = 0.494) under continuous sisal cultivation. Thus there had been a serious decline in soil fertility under sisal cultivation, and this decline was not adequately reversed by fallowing.