ABSTRACT: In the last 20 yr, an increasing abundance and northward expansion of Sardinella aurita has been reported in the western Mediterranean. The present study characterizes the spawning habitat of S. aurita at the northern limit of their geographic distribution and determines the hydrodynamic conditions that could control their northward expansion in the western Mediterranean. Data along the Catalan shelf and slope were obtained in 4 oceanographic cruises conducted in the summers of 2003 and 2004. A clear preference for spawning in coastal areas shallower than 100 m was determined, with larvae showing a wider distribution extending offshore up to the position of the 150 m isobath. The greatest abundance of eggs and larvae was found in the southern half of the area on the wide shelf near the mouth of the Ebro River. This zone was characterized by high values of surface chlorophyll a in association with relatively low salinity waters from the Ebro River. In the northern part of the area, a thermal front across the shelf, with higher sea surface temperatures on its southern side, marked the northern limit of egg and larvae distribution. The northern side of this front was under the direct influence of the shelf–slope current, advecting slope waters from the north, while its southern side was dominated by coastal water. Larvae carried by coastal waters tended to concentrate near the convergence associated with the front. Thus, the thermal front retains and concentrates S. aurita larvae, and could represent one of the limiting factors for the expansion of the species towards the north.