When the Argonauts landed at Cios, Hylas went to fetch water from a spring. There a water-nymph, overcome by his beauty, kidnapped him and took him into the spring. Heracles and Polyphemus searched too long for Hylas, and the Argo sailed without them.
Hylas was the son of Theiodamas, King of the Dryopians. Heracles killed Theiodamas in battle, but spared young Hylas and took him into his household. Hylas became Heracles' devoted companion for many years and accompanied him on the voyage of the Argo.
Hylas is portrayed on coins from Kios and on frescoes in Herculaneam and Pompeii; for instance in the Casa del Efebo in Pompeii.