We isolated a novel yeast α‐COP mutant, ret1‐3, in which α‐COP is degraded after cells are shifted to a restrictive temperature. ret1‐3 cells cease growth at 28 C and accumulate the ER precursor of carboxypeptidase Y (p1 CPY). In a screen for high copy suppressors of these defects, we isolated the previously unidentified yeast ϵ‐COP gene. ϵ‐COP (Sec28p) overproduction suppresses the defects of ret1‐3 cells up to 34 C, through stabilizing levels of α‐COP. Surprisingly, cells lacking ϵ‐COP (sec28Δ) grow well up to 34 C and display normal trafficking of carboxypeptidase Y and KKXX‐tagged proteins at a permissive temperature. ϵ‐COP is thus non‐essential for yeast cell growth, but sec28Δ cells are thermosensitive. In sec28Δ cells shifted to 37 C, wild‐type α‐COP (Ret1p) levels diminish rapidly and cells accumulate p1 CPY; these defects can be suppressed by α‐COP overproduction. Mutant coatomer from sec28Δ cells behaves as an unusually large protein complex in gel filtration experiments. The sec28Δ mutation displays allele‐specific synthetic‐lethal interactions with α‐COP mutations: sec28Δ ret1‐3 double mutants are unviable at all temperatures, whereas sec28Δ ret1‐1 double mutants grow well up to 30 C. Our results suggest that a function of ϵ‐COP is to stabilize α‐COP and the coatomer complex.