Theileria parva is a tick-transmitted protozoan parasite that infects and transforms bovine lymphocytes. We have previously shown that Theileria parvaChitongo (TpC) is an isolate of lower virulence than T.parvaMuguga (TpM). Lower virulence appeared correlated with a delayed onset of thelogarithmic growth phase of TpC-tranformedperipheral blood mononuclear cells after in vitro infection. In the current study, infection experiments of WC1(+)-gammadelta-T cellsrevealed thatonly TpMcould infect these cells andthat no transformed cells could be obtained withTpC sporozoites. Subsequent analysis of susceptibility of different cell lines and purified populations of lymphocytes for infection and transformation by both isolates showed that TpMsporozoites could attach and infectCD4(+), CD8(+) and WC1(+) T lymphocytes, but that TpCsporozoites were only observed to bind to the CD8(+) T cell population. Flow cytometry analysis of established,transformed clones confirmed this bias in target cells. TpM-transformed clones consisted of different cell surface phenotypes, suggesting that they were derived from either host CD4(+), CD8(+)or WC1(+)T cells. In contrast, all in vitro and in vivoTpC-transformed clones expressed CD8 but not CD4 or WC1, suggesting that the TpC-transformed target cells were exclusivelyinfected CD8(+) lymphocytes.So a role of cell tropism in virulence is likely. Since the adhesion molecule p67 is 100 % identical between the two strains, a second, high affinity adhesin that determines target cell specificity appears to exist.