A study of dialectal variation in Mexican Sign Language (MSL), the primary language for a large segment of Mexico's deaf community, is presented. Signs used by nine different sources representing various locations, ages, and social groups are compared. The first section reviews a number of previous informal assessments of dialectal variation in MSL. The second section describes the study, in which individuals in five locations (two in Hermosillo, one in Cuernavaca, and two in Mexico City) were asked to provide signs for words from a list of 100. In addition, four published signing dictionaries were consulted. Identical, similar, and variant signs were then analyzed and compared with a similar analysis of the same 100 words with native English-speakers from the United States with different accents. Results are interpreted in the third section. It is concluded that the majority of the evidence suggests MSL is a single language, at least for the signers and dictionaries sampled. The moderate amount of dialectal variation is seen as not hindering effective communication. Further research on mutual intelligibility is recommended. The greatest variation appeared in the Monterrey dialect sample; explanations are offered. Implications of the findings for language planning are discussed. (MSE)