Ballan wrasse (Labrus bergylta) is a specialized cleaner fish used in salmon farms as a biological treatment against sea lice. Its commercial rearing is at present mostly experimental. A number of key aspects, including the molecular and physiological mechanisms that promote its growth and development, are still largely unexplored. In this study histological, biometric, biochemical and molecular approaches are combined for the first time to investigate the changes in growth (insulin-like growth factor 1 and 2 and myostatin) and stress (heat shock protein 70 and cortisol) markers that occur during ballan wrasse larval development by relating them to larval stages and feed changes. The real-time PCR data demonstrated that igf1 transcripts rose from 1 day post-hatching (dph) and were no longer detectable 38 dph, whereas igf2 and myostatin transcripts were low and stable until 28 dph, then rose in late larval stages. The biometric and histological data matched the molecular findings, documenting rapid growth and development of the larval digestive tract and assimilation ability. Cortisol was lowest at hatching, it rose slightly at first feeding, and then increased during larval development; a similar trend was detected for hsp70 gene expression. The low cortisol levels found at the earliest larval stages reflect a poor stress-coping ability, a feature that may actually protect larvae from the elevated metabolic demands involved by stress responses and promote faster growth and survival. The present data can be applied to improve the rearing performances of this important cleaner species and reduce captures from the wild.