We have screened a collection of transposable-element-induced mutations for those which dominantly modify the extra R7 phenotype of a hypomorphic yan mutation. The members of one of the identified complementation groups correspond to disruptions of the tramtrack (ttk) gene. As heterozygotes, ttk alleles increase the percentage of R7 cells in yan mutant eyes. Just as yan mutations increase ectopic R7 cell formation, homozygous ttk mutant eye clones also contain supernumerary R7 cells. However, in contrast to yan, the formation of these cells in ttk mutant eye tissue is not necessarily dependent on the activity of the sina gene. Furthermore, although yan mutations dominantly interact with mutations in the Ras1, Draf, Dsor1, and rolled (rl) genes to influence R7 cell development, ttk mutations only interact with yan and rl gene mutations to affect this signaling pathway. Our data suggest that yan and ttk both function to repress inappropriate R7 cell development but that their mechanisms of action differ. In particular, TTK activity appears to be autonomously required to regulate a sina-independent mechanism of R7 determination.