The Mediterranean gobiids Didogobius splechtnai and Gammogobius steinitzi occur sympatrically in submerged caves at the Balearic Island of Ibiza (Western Mediterranean). Both species inhabit the dark backgrounds of large caves, G. steinitzi mainly the walls and D. splechtnai the sediment-covered bottoms. Morphological characters like body proportions, meristics, the lateral-line system and osteological features are described and compared.