The morphology of retinal ganglion cells projecting to the superior colliculus (SC) of the thirteen-lined ground squirrel (Spermophilus tridecemlineatus) was studied after retrogradely labeling the cells with cholera toxin subunit B. On the basis of previous reports, labeled cells were classified as small (6-10 microm in soma diameter), medium (11-14 microm), or large (>14 microm). A total of 3,427 cells were studied. Small cells constituted 78% of the population, 21% were medium cells, and only 1% were classified as large. The morphology of medium-sized cells was studied in more detail because large cells were few in number and the staining of the dendritic tree of small cells was not optimal. The best labeled medium-sized cells were classified on the basis of the shape and size of their dendritic tree and the pattern of dendritic ramification. Four types were identified among the medium-sized ganglion cells. Two types were classified as symmetric delta-like and asymmetric delta-like cells considering the relative symmetric or asymmetric distribution of their dendritic branches and their similarities with the delta type of the cat. Approximately 52% of all the medium-sized cells studied were symmetrical delta-like, and 19% were classified as asymmetrical delta-like. These cells were also very similar to the symmetrical and asymmetrical directionally selective ganglion cells described in rabbit retina. Other cells were termed beta-like. They had the smallest dendritic tree diameter, and their tree size seemed to be related to retinal eccentricity. Medium beta-like cells comprised approximately 21% of all cells projecting to the SC. The fourth type was termed "acute angle" because most of their dendritic branches were relatively straight and formed acute angles (10-45 degrees) at their branching points. These cells were few in number (approximately 8% of all medium-sized cells studied) and did not resemble any reported previously in cats. Thus, a variety of morphological types of retinal ganglion cells projected to the SC. Of these, the symmetrical and asymmetrical delta-like cells appeared to correspond to the directionally selective type described in the ground squirrel (Michael, C.R. [1968] J. Neurophysiol. 31:257-267) and reported in the rabbit retina.