Restriction fragment length polymorphisms (RFLPs) were used as markers to determine the transmission of chloroplast DNA (cpDNA) in poplar crosses. The plant material studied included individual trees ofPopulus trichocarpa, P. maximowiczii xtrichocarpa, P. maximowiczii xnigra, and offspring from controlled crosses between these trees. RFLPs were identified by direct observation of stained restriction fragments, as well as by molecular hybridization with heterologous cpDNA probes. Analysis of the restriction fragment patterns in the parents and their progeny showed only the patterns of the maternal tree in the progeny, while no paternal type was found. These results provide clear evidence of a maternal mode of chloroplast inheritance in the poplar clones studied.