Many deaf children and adults show lags in mathematical abilities. The current study examines the basic number representations that allow individuals to perform higher-level arithmetical procedures. These representations are normally present in the earliest stages of development, but they may be affected by cultural, developmental, and educational factors. Deaf and hearing participants were asked to perform two number comparison tasks. Analysis of response times revealed that all participants showed effects normally associated with representation of magnitude on a visual-analog mental number line: SNARC, distance, and size effects. However, deaf participants were slower overall in making comparative judgements, suggesting that whilst their numerical representation does not differ from that of hearing individuals, the efficiency with which they process basic numerical information is lower. The results are discussed in terms of interactions between biologically determined numerical representations and cultural and schooling factors that differentially affect deaf and hearing individuals.