Abstract Fragments of Y chromosomes were induced by irradiating attached-X Drosophila melanogaster females carrying a doubly marked Y chromosome, and detected by loss of one of the Y markers. Slightly more than half of the 55 fragments recovered showed linkage of fourth chromosome markers, implying a high frequency of association of Y and fourth chromosomes in oocytes. Where such fragments were shown to have Y centromeres, a high incidence of non-disjunction from the attached X was found, suggesting that exchange between heterologues leads to their separation at Anaphase I, and frequent non-disjunction from their regular pairing partners. Lucchesi 's 9 assumption that fragments are not pairing-dependent for their formation is thus invalidated. It is suggested that non-reciprocal exchanges induced between the two Y arms may account for other fragments, these likewise being pairing-dependent. There is as yet no evidence that any exchange process occurs with appreciable frequency in oocytes in stage 7 or earlier that is not dependent on regular, frequent associations of the chromosomes involved.