In xenopus embryos at stages 28-32 one quarter to one third of the left eye rudiment was replaced by a similarly sized piece from a different position in a right eye rudiment. Three groups of operations were performed: (1) temporal tissue was placed in a nasal position; (2) nasal tissue was placed temporally; (3) ventral tissue was placed dorsally. The visuotectal projections made by these 'pie-slice' compound eyes were assessed electrophysiologically at 1 week to 6 months after metamorphosis. Of 97 animals, 71 yielded interpretable projections. In most cases two projections could be identified in each map. One, ascribed to the host part of the retina, extended over the entire tectal surface mapped. The other, identified as that from tissue derived from the pie-slice graft, projected to the tectum in register with that part of the host retina which matched the pie-slice in origin. Both projections were well ordered, and in the orientation expected if the corresponding piece of retinal tissue had participated in a normal projection. Consistent differences in pie-slice size and tectal coverage between the three groups were found. Pie-slices of nasal origin gave maps showing that they came from a relatively large portion of the retina and projected to a relatively large amount of the tectum; those of temporal origin occupied relatively small amounts of field and tectum. It was concluded that these results are further evidence for the existence of positional markers in the retina which are used for the assembly of the retinotectal map.