In this paper we report on our attempt to teach the polyglot savant Christopher (‘C’ hereinafter) British Sign Language (BSL). BSL presents C with a novel challenge in the use of hand-eye coordination, while at the same time offering him the linguistic ingredients he is obsessed with. Despite his deficits in key areas of intellectual ability, communication skills and visuo-spatial cognition, C has developed a working knowledge of BSL through processes of circumvention, adaptation and invention. As a form of control, we taught BSL to a comparator group of talented second-language learners. We do not discuss this comparison in depth here (see Morgan et al. in preparation) but refer to some of the test scores as a guide to how normal a sign learner C is.Results from formal tests of Cʼs linguistic knowledge, and observational study of his developing communicative ability in BSL, are analysed and described. These results illuminate the structure and use of BSL, highlighting the important role of visuo-spatial cognition in its acquisition and manipulation.Our findings support the assumption that the organisation of knowledge of language is largely modality independent, whereas the exploitation of specific grammatical devices is language and modality dependent. C has attained a certain level of linguistic competence in BSL, and his performance in the language is largely in conformity with his previously established mixed profile of abilities and disabilities.