The frequency of persistence of three Staphylococcus epidermidis, Staphylococcus haemolyticus and Staphylococcus saprophyticus strains, respectively, was studied in BALB/c mice at the 10th day of intraperitoneal (ip) challenge. 245 out of 416 mice survived after infections with four bacterial suspensions of different colony forming units (CFU) of each strain. Staphylococci persisted in 61 mice (24,9%). The main sites of persistence were the kidneys, while cocci were rarely isolated from the spleen and the liver. S. epidermidis persisted with a significantly higher rate than the other two species, because S. epidermidis in 28,8%, S. haemolyticus in 4,9%, and S. saprophyticus in 3,6% were reisolated from the organs of the respective infected and surviving animals. The organ persistence was proportional to the amount of bacteria injected. The persistence resulted in subacute microabscesses in the organs. Reisolates of persisting bacteria remained stable in phenotype and genotype concerning antibiotic resistance patterns and biochemical activities for the taxonomic implication, whereas cell surface properties characterizable with phage types altered considerably during persistence. It is concluded that cocci of all three Staphylococcus species are invasive and can persist to a certain extent in the organs of animals with normal immune system, too, after artificial inoculation into the peritoneum i. e. to the serosal surfaces.