This dataset contains information of 11 morphometric variables measured in 56 specimens of Heterospio variabilis, an annelid polychaete belonging to the Family Longosomatidae, and the geographical position where they were found. The biological material was collected during three oceanographic expeditions carried out between May 1980 and August 1985. The examined specimens were collected in 12 stations from the western continental shelf of the Gulf of California, between the 20.893° to 31.33° North latitude and -105.4583° to - 114.3866° West longitude. The samples were taken with a Van Veen (0.063 m 2 ) or Smith-McIntyre (0.1 m 2 ) grabs and the specimens were fixed with 4% formaldehyde, preserved in 70% ethanol and deposited in the Colección Nacional de Anélidos Poliquetos, ICML-UNAM. Fifty-six specimens were examined with a stereoscope and compound microscope and, using an eyepiece micrometer, 11 morphologic variables were measured; with exception of the number of branchiae and the rate between length of chaetiger 9 and anterior region, all units are in mm: total length (Tl) standardized to the length back to the chaetiger 12; number of branchiae (Bn); length (Pl) and width (Pw) of prostomium; length of chaetiger 1 to chaetiger 8 (CH1-l–CH8-l); width body (at chaetiger 5 without parapodia) (Aw); length of chaetiger 9 (CH9-l); length of chaetiger 10 (CH10-l), 11 (CH11-1) and 12 (CH12-1); and the rate between length of chaetiger 9 and length of tip of prostomium to chaetiger 8 (RCH9/Al)