The only non-human host of Lassa virus so far identified is the multimammate mouse, Praomys (Mastomys) natalensis, but its precise role in the natural Lassa fever cycle remains to be determined. This species is also an important link in the plague cycle in southern Africa and is one of the commonest rodents of Africa. It is a prolific breeder and can be kept and bred easily in captivity. It is thus an excellent laboratory animal, although it needs to be handled with care because it is aggressive towards man and bites readily. The current status of knowledge of its taxonomy, ecology, distribution, and role as a disease vector is reviewed, but attention is drawn to the possibly disastrous consequences of attempting to eradicate a vector species before the natural cycle of the disease and the ecology of the vector are fully understood.