Alcyonium acaule (Cnidaria, Octocorallia) is a common, hard-bottom soft coral in the northwestern Mediterranean Sea. This study describes sexual reproduction and the gamete development cycle of this soft coral. A population at 15-18 m depth in the Marine Protected Area of the Medes Islands (42o02′55″ N, 3o13′30″ E) was sampled from July 1994-August 1995. A. acaule is gonochoristic and a surface brooder, spawning once a year in early summer. The mean diameter of ripe spermatic sacs was 400 ± 91 (SD) μm, and the mean diameter of mature oocytes was 473 ± 37 (SD) μm. There were 30 spermatic sacs polyp-1 in males and 14 oocytes polyp-1 in females. Different phases of gametogenesis in female and male colonies were examined separately with respect to seasonal changes in bottom temperature and solar irradiance. The data suggest that the relatively constant temperatures in January-April are probably not related to oocyte maturation, but that rising temperatures in May could affect sperm maturation. Rapidly increasing solar irradiance in March may be the trigger for vitellogenesis and oocyte maturation, although the mechanism for this in anthozoans is not understood. © 2012 Springer-Verlag Berlin Heidelberg.