Peripheral expanded T cell clones have been discussed mainly in relation to certain diseases or immune function in humans and mice. There is little information on their ontogeny, stability and distribution among T cell subsets as well as major lymphoid organs. We applied reverse transcription-polymerase chain reaction (RT-PCR) with family specific primers for monkey T cell receptor β chain V regions and single-strand conformation polymorphism (SSCP) analysis to analyze the expanded T cell clones in cynomolgus monkeys ( Macaca fascicularis). A number of expanded T cell clones were detected in the peripheral blood of young and adult monkeys, but few expanded T cell clones were detected in the blood of a fetus and a 2-day-old neonate. The clones in adults were maintained over 3 months. These expanded T cell clones were distributed only in peripheral blood and spleen, but few were found in lymph nodes (axillary, inguinal and intestinal). The number of expanded T cell clones was much greater in CD8 single-positive (CD8sp) T cells than in CD4sp T cells, showing that most of these clones originated in the CD8sp T cell population. Almost all the expanded CD8sp T cell clones belonged to the CD28 −, CD29 hi and Fas + subset. The usage of V β genes was not skewed in the 24 V β. Furthermore, higher mRNA signals for effector molecules perforin and IFN-γ were detected in CD8sp T cell subsets with phenotypes of CD28 −, CD29 hi and Fas +, suggesting that the expanded T cells might have developed in relation to T cell activation in the periphery of cynomolgus monkeys.