Trypanosomatid parasites provide an extreme model for the posttranscriptional control of eukaryotic gene expression. However, most analysis of their differential gene regulation has focussed on comparisons between life-cycle stages that exist in the blood of mammalian hosts and tsetse flies, the parasite's vector. These environments differ acutely in their temperature, and nutritional, metabolic and molecular composition. In the bloodstream, however, a more exquisitely regulated developmental step occurs: the production of transmissible stumpy forms from proliferative slender forms. This transition occurs in the relatively homogenous bloodstream environment, with stumpy-specific gene expression being repressed until accumulation of a proposed parasite-derived signal, stumpy induction factor. Here, we have dissected the regulatory signals that repress the expression of the stumpy-specific surface transporter PAD1 in slender forms. Using transgenic parasites capable of stumpy formation we show that PAD1-repression is mediated by its 3'-untranslated region. Dissection of this region in monomorphic slender forms and pleomorphic slender and stumpy forms has revealed that two regulatory regions co-operate to repress PAD1 expression, this being alleviated on exposure to SIF in pleomorphs or cAMP analogues that act as stumpy induction factor mimics in monomorphs. These studies identify elements that regulate trypanosome gene expression during development in their mammalian host.