A female sunset thick-lipped gourami (Colisa labiosa) weighing 8 grams and measuring 5.4 cm standard length, was presented with a 0.5 cm diameter midventral swelling of sudden onset 1 month previously. The size and position precluded a simple excision and a diagnostic plan was pursued to define the mass further prior to surgery. Cytology from a fine needle aspirate revealed only degenerated red blood cells free nuclei and thrombocytes. Survey radiographs (using dental film), a barium gastro-intestinal study, and doppler ultrasonography demonstrated that the mass was not attached to the gastro-intestinal tract, that it communicated with the coelomic cavity, and that a major blood supply serviced the mass at its cranio-dorsal aspect. Anesthesia was induced and maintained with tricaine methane-sulfonate, initially by bath treatment and then by continuous flow over the gills. Total anesthesia time was 47 minutes. The fish was positioned in dorsal recumbency under a surgical microscope. Using microsurgical techniques the mass was excised and the muscle layer repaired. Recovery was uncomplicated and the fish has continued to do well. On histopathology the mass was diagnosed as a consolidated hematoma, etiology undetermined. This case illustrates that small patient size need not be a deterrent to a diagnostic work-up and surgical therapy in fish.