The fisheries data supplied to the Food and Agriculture Organization of the United Nations (FAO) by national agencies have served as the primary tool for many global and regional studies. However, it is recognised that these data are incomplete and often underestimate actual catches, particularly for small-scale fisheries. This study reconstructed total marine fisheries catches from 1950 to 2005 for Mozambique and the United Republic of Tanzania, by applying an established catch reconstruction approach utilising all available quantitative and qualitative data, combined with assumption-based estimations and interpolations. Since the 1950s, Mozambique has reported primarily industrial catches and has substantially under-reported the country's small-scale fishing sector due to lack of resources and civil war. In Tanzania, Zanzibar's recorded fisheries statistics prior to 2000 are absent from Tanzania's marine fisheries catches reported to FAO, and total mainland catches are at least one-third larger than officially reported. Based on our reconstruction, since 2000, Mozambique caught between 150 000 and 172 000 t y-1, while the United Republic of Tanzania caught at least 95 000 t y-1. For the period 1950-2005, reconstructed total marine catches were 6.2 and 1.7 times greater than data supplied to FAO by Mozambique and Tanzania respectively. The reliance on incomplete and substantially under-reported national data puts authorities under serious risk of over-licensing fishing access and mismanaging marine ecosystems and national food security.