Sixteen Finnish strains of Rubus arcticus L. ("Arctic bramble" or, as proposed by Miss A. Ryynanen, nectarberry) were tested for self-fertility; they turned out to be self-sterile as a rule. The rich berry production in certain regions in Finland is not due to self-fertility, because all strains obtained from those regions showed clear self-sterility. Double pollinations revealed that the self-sterility is due to incompatibility. The results from pollinations between parents and sibs fit neither into the gametophytic-gametophytic incompatibility system nor into the one locus system with the oppositional type of sporophytic-sporophytic control, but rather into a sporophytic-gametophytic system. There are at least five incompatibility alleles involved. Pollinations within a clone were more strictly sterile than pollinations within an incompatibility equivalence class. Autotetraploids appeared to be more self-fertile than diploids.Self-sterility, which is fairly widely distributed in the Rosaceae, has been reported for the following species in the genus Rubus: R. odoratus L. (Subgenus Anoplobatus), R. allegheniensis POR-TER and R. trivialis MICHX. (both in the subgenus Eubatus) from North America, R. itlmifolius