We explored molecular alterations in the developmental switch from the L3 to the exsheathed L3 (xL3) and to the L4 stage of the barber’s pole worm (Haemonchus contortus) using an integrated proteomic, transcriptomic and bioinformatic approach. About 16% of 9842 transcripts in the transcriptome were expressed as proteins (n = 1596). There was a positive correlation (r = 0.39-0.44) between transcription and protein expression for distinct developmental stages of the nematode. There was a negative correlation (r = -0.6 to -0.5) in the differential protein expression between developmental stages upon pairwise comparison. Changes in protein expression from the free-living to the parasitic phase of H. contortusrelated to enrichments in biological pathways associated with metabolism (e.g., amino acid metabolism and carbohydrate and lipid degradation), environmental information processing (e.g., signalling and interactions) and/or genetic information processing (e.g., translation). Fatty acid degradation and steroid hormone biosynthesis were suppressed, whereas translation and protein processing in the endoplasmic reticulum were upregulated in the transition from the free-living L3 to the parasitic larval stages of the nematode. Post-transcriptionalregulation was inferred to elicit these alterations, with particular miRNAs likely having roles in environmental adaptations and/or stress responses during developmental transitions. The results of this study provide a comprehensive insight into the developmental biology of this economically important worm at the molecular level, and the methodologies used are readily applicable to other parasites.