A deterministic model was constructed for studying the circulation of Feline Immunodeficiency Virus (FIV), a feline retrovirus homologous to Human Immunodeficiency Virus (HIV), within populations of domestic cats. The model has been tested with data generated by a long-term study of several natural cat populations. Simulations and a study of stability show that once introduced, the retrovirus is maintained within the population, with a stable equilibrium stage reached by both numbers of susceptible and infected individuals. An estimation of parameters indicates that the transmission rate is low and depends on the structure of the population. In addition, FIV has a low impact on the population in that the total number of cats at equilibrium when this virus is present is almost always equal to the habitat carrying capacity in the absence of the virus. Those results, in agreement with other observations, suggest that FIV originally arose in the distant past.