Under hot, dry conditions in the Central Valley of California, the development of brown rot in stone fruits was found to be closely associated with the presence of Nitidulid beetles and fruit injury caused by Cydia (Grapholitha) molesta (Busck) or with the natural splitting of the fruit endocarp, or both. During 1971-73, on decaying stone fruits on orchard trees, Carpophilus freemani Dobson predominated in June, July and August; populations of C. mutilatus Erichs. and Haptonchus luteolus (Erichs.) were rare until August, but increased and became prevalent in September. Throughout the summer, C. hemipterus (L.) was present in very small numbers in the trees. In decaying fruit on the orchard floor, C. mutilatus was less abundant than the other species. C. mutilatus and H. luteolus were important vectors of Scherotinia (Monilinia) fructicola, which causes brown rot, in late-maturing peach and nectarine cultivars. They were active visitors of the exit holes of Cydia molesta in healthy fruit, they were contaminated in nature with viable conidia of S. fructicola and they transmitted the spores to injuries in fungicide-treated or untreated fruit. Although Carpophilus freemani and C. hemipterus experimentally transmitted S. fructicola and were contaminated in nature with viable conidia, they were not important vectors since neither actively visited injured healthy fruit in furrow-irrigated orchards. Olfactometer tests showed that C. freemani preferred decaying to healthy fruit odours, whereas the other species preferred the latter. Moisture was an important factor influencing the attraction of all species to injured fruit. These data show that two species of Nitidulid beetles, C. mutilatus and H. luteolus, are effective vectors of S. fructicola in late- maturing stone fruit cultivars with injured fruit and may explain why recommended fungicide applications sometimes fail to control fruit decay in California orchards. Control of brown rot or fruit in the field was obtained by insecticide sprays designed to reduce damage by Cydia molesta.