Anteroposterior asymmetries are apparent in C elegans development before the first cell division. Here we identify the cue that specifies the anteroposterior axis, and investigate how this cue is interpreted to generate initial asymmetry. In C. elegans, the sperm normally enters the egg in an invariant position. We have found that causing fertilisation to occur in the abnormal end of the egg completely reverses the orientation of the anteroposterior axis, but gives otherwise normal development. This result suggests that a component of the sperm normally specifies the anteroposterior axis. We have found that a cytoplasmic rearrangement in the uncleaved zygote is directed by the sperm, suggesting a mechanism by which the sperm may specify the axis. The results additionally reveal that the C elegans oocyte is constructed with no axis prespecified in the form of asymmetrically localised cytoplasmic determinants.