We evaluated inter- and intraspecific variations of the intestinal coiling patterns and intestinal length of the oily bitterlings, Tanakia limbata, Tanakia koreensis, Tanakia somjinensis, and Tanakia signifer. All the oily bitterling species shared a unique intestinal coiling pattern, expressed as an oval shaped loop. The ratio of the intestinal length to standard length varied among species, viz., both T. koreensis and T. signifer have significantly longer intestines than T. limbata and T. somjinensis. The intestinal length may be a useful diagnostic character between two sibling species, T. koreensis and T. somjinensis, although we observed intraspecific variation in the intestinal length of T. limbata and T. koreensis among collection sites.