In a new mouse mutant, circletail (Crc), failure of neural tube closure (embryonic day [E] 8-9) is associated with errors in retinal axon projection at the optic chiasm (E12-18), such that many axons normally projecting contralaterally instead grow to ipsilateral targets. Although the architecture of the chiasmatic region is altered, neurons and glia containing putative cues for axon guidance are present. The aberrant ipsilateral-projecting cells originate from a nonrandom expansion of the wild-type uncrossed retinal region. These axon pathway defects are found in two other mutants with cephalic neural tube defects (NTD), loop-tail (Lp) and Pax3 (splotch; Sp(2H)). Crc is phenotypically similar to Lp, exhibiting an open neural tube from midbrain to tail (craniorachischisis), while splotch has spina bifida with or without a cranial NTD. The retinal axon abnormalities occur only in the presence of NTD and not in homozygous mutants lacking cranial NTD. Thus, failure of neural tube closure is associated with failure of many retinal axons to cross the ventral midline. This study therefore reveals an unexpected connection between closure of the neural tube at the dorsal midline and development of ventral axon tracts. genesis 27:32-47, 2000.