A number of Pitymys duodecimcostatus males from a natural population were treated with hydroxyurea, which produces a gap in the spermatogenic line, to study the synaptic sequence of meiotic prophase. The use of substaging criteria, based on the morphological characteristics of the meiotic cells themselves, gave rise to a classification scheme consistent with the chronological sequence revealed by hydroxyurea treatment, which supports the usefulness of these independent criteria. The characteristics of meiotic prophase in P. duodecimcostatus are also described and discussed.