A consistent message emerges from research on undergraduate students' perceptions of assessment which describes traditional assessment as detrimental to learning. However this literature has not included students in the pure sciences. Mathematics education literature advocates the introduction of innovative assessment at university. In this literature however students' voices tend to be unheard. We investigate mathematics students' perceptions of assessment at one high-ranking UK University. We find that, in contrast to the message from the general literature, students perceive traditional assessment as the best discriminator of ability. We suggest that the drive towards the introduction of innovative assessment in mathematics should be considered in the light of students' perceptions and that further research is needed to investigate the origins of those perceptions as well as how those perceptions may be influenced by the subject of study.