This article is originated from research project of rodentsMultimammate mice are the most important vertebrate pests in Sub-Saharan Africa and are also reservoirs of many zoonotic diseases, including sylvan plague. This study investigated the population dynamics and breeding patterns of this mouse in irrigated rice cropping systems in eastern Tanzania. RESULTS:Themultimammatemouse,Mastomysnatalensis,populationvariedwithhabitatandmonths.Fallowlandhada more abundant population than rice fields. The highest population peak was observed during the dry season from July to October. Mastomys natalensis is sexually active throughout the year in the study area, although it reaches the highest level in June and December when rice is at the maturity stage. This suggests that breeding is highly influenced by the presence of a rice crop in both seasons. More juvenile individuals were recorded in August and September, indicating that they were produced in the previous breeding months. The sex ratio of M.natalensis was not skewed to either males or females, indicating that it was at parity. CONCLUSION: Rodent population dynamics during the study periods in all habitats indicated that high birth rates accounted for the rapid population growth and turnover. Regular control and sustainable operations are thus essential if rodent pest populations are to be kept with in tolerable limits.This work was supported by the Zonal Agricultural Research and Development Funds (ZARDEF), Eastern Zone. The authors appreciate the excellent field assistance from Khalid S Kibwana, Omary Kibwana, Shabani Lutea, Geoffrey Sabuni and Ramadhani Kigunguli of the Pest Management Centre, Sokoine University of Agriculture Morogoro,Tanzania.