Belg. J. Zool., 135 (supplement) : 159-165Rodent population fluctuations and breeding patterns were investigated at localities in South-west, Central and North-east Tanzania. The three localities are ecologically heterogeneous in vegetation types, rodent species diversity, rainfall pattern and altitude. Capture-Mark-Release studies were conducted in 2001-2003 to com- pare rodent species composition and population trends. In North-eastern Tanzania, species composition is diverse and includes Mastomys natalensis, Lophuromys flavopunctatus, Grammomys dolichurus, Arvicanthis nairobae, Praomys delectorum and Mus sp.. Five species were recorded in South-western Tanzania namely, M. natalensis, Graphiurus murinus, Saccostomus elegans, Tatera leucogaster and Steatomys pratensis. In Central Tanzania M. natalensis was dominant, but a few Lemniscomys griselda were captured. Rodent abundance fluctuations were dis- tinctively seasonal, especially for M. natalensis in the three localities and T. leucogaster in South-west Tanzania. In North-eastern Tanzania, L. flavopunctatus, G. dolichurus, A. nairobae and P. delectorum had low, but relatively sta- ble populations throughout the year. In South-west Tanzania, population peaks of M. natalensis and T. leucogaster were reached in the dry season (June-September). In Central Tanzania, breeding of M. natalensis was seasonal, with highest population abundance during July-November. Female M. natalensis were reproductively active in January- May and males had scrotal testes in December-June. No males were sexually active during July-November. Female T. leucogaster in South-western Tanzania were reproductively active during November-April/May whereas sexually active males appeared in the population during November-March. In view of the observed rodent population fluctu- ations and breeding patterns, recommendations are given for pragmatic rodent control in South-west and Central Tanzania and for plague in North-eastern Tanzania