Differences in gene expression between adult and immature Fasciola hepatica (liver fluke) parasites isolated from the mammalian host were investigated using the technique of differential display. For any given primer combination used to produce these displays there were, on average, 22% apparently adult-specific and 14% apparently immature-specific cDNA products able to be identified, consistent with a high degree of differential gene expression between these two parasite developmental stages. Several cDNA fragments specific to immature parasite RNA were isolated and cloned. An abundant 400- to 500-bp RNA species was identified on a Northern blot by hybridization to the cloned DD2 cDNA fragment and was determined to be expressed at levels at least 10-fold higher in immature parasites relative to adult parasites. mRNA transcripts corresponding to the remaining cDNA fragments (DD14, DD16, DISP10, and DISP2) were apparently expressed at levels below the sensitivity limits of Northern analysis, although differential expression of these transcripts was confirmed by reverse transcriptase PCR (RT-PCR). The identities or functional significance of each of the five differentially expressed cDNAs identified in this study is still unclear due to the lack of any significant sequence similarity to the entries currently held within sequence databases.