Two experiments compared the SNARC effect for calendar months (January-December) in 16 normal controls against four participants reporting a common but little-studied variety of synaesthesia where ordinal sequences are explicitly experienced in elaborate spatially extended patterns (spatial forms). The SNARC effect (spatial-numerical association of response codes) (Dehaene et al., 1993) in which responses to early versus late members of ordinal sequences show left-hand versus right-hand reaction time (RT) advantages, respectively, has previously provided evidence for implicit associations between sequential and spatial representation in non-synaesthetes (Gevers et al., 2003). The current study revealed an automatic month-SNARC effect for the synaesthetes, with the left/right-hand advantage reversing for synaesthetes who experienced early months on the right rather than the left of their roughly circular year forms. The absence of any month-SNARC effect among 16 controls demonstrated cognitive differences in sequence representation between controls and synaesthetes, but failed to replicate previous findings for non-synaesthetes. Certain details of the synaesthetes' SNARC effect may also constrain the way SNARC effects in non-synaesthetes are interpreted.