A circannual rhythm of testicular size existed in California ground squirrels(Spermophilus beecheyi)kept at 20°C under 3 regimes of daylength: Santa Barbara (SB), LD 12:12, and southern hemisphere (SH). Seasonal descent of testes was measured on a scale of 0–4. The squirrels on SB daylength maintained a cycle of descent of 11.13 months; those on LD 12:12 of 11.61 and those on SH daylength 10.65. The latter 2 groups showed great variation. The persistence of the rhythm under constant daylength (LD 12:12) and also on a 6‐months phase shift indicates that the rhythm is circannual.