Photosynthate partitioning was examined in seedings of sour orange (Citrus aurantium L.) and Carrizo citrange (Poncirus trifoliata [L.] Raf. x C. sinensis [L.] Osbeck) grown with split root systems inoculated on one side with vesicular-arbuscular mycorrhizal fungus (Glomus intraradices Schenck and Smith). Source-sink relations were studied without mitigating differences in mineral content or physiological age that can occur in separate plant comparisons, because phosphorus was evenly distributed between leaves on opposite sides of the seedlings. Above-ground portions of each plant were exposed to (14)CO(2) for 8.5 minutes and ambient air for 2 hours, followed by extraction and identification of labeled assimilates. Mycorrhizal halves of root systems accumulated 66 and 68% of the (14)C-labeled photosynthates translocated to roots of sour orange and ;Carrizo' citrange, respectively, as well as an average of 77% greater disintegrations per minute per gram fresh weight. Distribution of (14)C-labeled assimilates was independent of phosphorus effects on photosynthate partitioning in leaves and did not reflect fresh or dry weights of roots or degree of mycorrhizal dependency of the species. Differences in radioactivity between mycorrhizal and nonmycorrhizal root halves after 2 hours indicated at least 3 to 5% of the whole plant (14)C-labeled photosynthates were allocated to mycorrhizae-related events on one side and that twice this amount, or 6 to 10%, might be expected if the entire root system was infected.