The oldest record of the brachyuran Diaulax Bell, 1863 is based on the description of Diaulax rosablanca n. sp.from the middle part of the Rosablanca Formation (Early Cretaceous, early Valanginian, Department of Santander, Colombia).This is also the oldest record for fossil crustaceans from Colombia.The new species is represented by numerous specimens showing ontogenic variation in size and shape, with adults becoming larger and wider at the anterior portion of carapace.Differences with the other five species of the genus are subtle but sufficient to erect a new species.The associated ammonites Lissonia riveroi (Lisson, 1907), Acantholissonia gerthi (Weaver, 1931) and Raimondiceras sp.confirm an age corresponding to the middle part of the early Valanginian.The paleoenvironment is interpreted as a shallow, restricted low-energy carbonate platform or lagoon within well-oxygenated warm waters.The outcrop includes other decapod remains, too incomplete to offer a preliminary identification.