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We continued our study on fusion and non-fusion reactions in the compound ascidian Botrylloides leachi with particular attention to the role of hemocytes in the induction of the strong cytotoxic reaction observed when genetically incompatible colonies encounter each other in cut surface assays. We used antibodies raised against Botryllus phenoloxidase (PO) and pro-inflammatory vertebrate cytokines for immunocytochemical and immunohistochemical analysis on contacting incompatible colonies in order to investigate cellular events during the non-fusion reaction. Results indicate that, analogously to Botryllus schlosseri, morula cells play a key role in the above reaction as they crowded inside the facing ampullae and entered the tunic where they degranulated and induced PO-driven cytotoxicity. Similarly to Botryllus, they also expressed molecules recognised by anti-IL-1-α antibodies, whereas the immunopositivity to anti-TNF-α was unexpectedly restricted to granular cells.