AbstractWe used a precision two-circle goniometer mounted to the stage of a compound microscope to determine the optical alignment and to measure the entrance aperture diameter of individual cuticular cones in the dioptric array of the lateral eye of juvenile horseshoe crabs in order to learn about the development of the visual field. Our results show that the extent of the visual field of juvenile horseshoe crabs with prosomal lengths about 20% ofadult size (14–21 mm) is about 70% that of the visual field of adult horseshoe crabs (prosomal lengths: 100+ mm). The visual field of such juvenile animals covers between 77 and 85 deg vertically and 140 and 145 deg horizontally. Assuming that the dioptric array is uniform and square packed, the average interommatidial angle of the juvenile animals is between 5.6 and 6.0 deg as compared to 4.6 deg for an adult animal. The diameter of the entrance aperture of individual cuticular cones increases markedly with increasing animal size. In addition, we noted a statistically significant trend for entrance aperture diameters to increase from anterior to posterior within the eye for animals of all sizes. There may be a slight trend for entrance aperture diameters to increase from dorsal to ventral within the eye. Our results indicate that the extent of the visual field and the resolution of the lateral eye approach adult values in advance of animals' reaching sexual maturity.