We identified to the lowest taxonomical level possible the preferred prey of Atlantic mackerel larvae from the southern Gulf of St Lawrence and assessed the extent to which prey selectivity varied within and among years. Mackerel larvae and their zooplankton prey were sampled in the summer of four consecutive years (1997–2000). The nauplii of the calanoid copepod Pseudocalanus sp. strongly dominated the diet of larvae <7 mm both in terms of numbers and carbon content, whereas larvae ≥7 mm mainly fed on fish larvae (including conspecifics) and cladocerans. Chesson’s alpha index revealed strong selectivity in all years for Pseudocalanus sp. nauplii in first-feeding larvae. Selectivity shifted to cladocerans and fish larvae around a body length of 7 mm. Intra- and inter-annual prey selectivity changes were mainly observed for alternative prey, during the period surrounding the shift in diet from small to large prey. Our results underscore the importance of considering the availability of the main prey Pseudocalanus sp. nauplii (early larval stage) as well as cladocerans and fish larvae (late larval stage), rather than the entire prey field in the future assessment of the role played by prey availability on larval mackerel vital rates.