The purpose of this study was to determine whether core temperature (T_c) telemetry could be used in ambulatory women to track changes in the circadian T_c rhythm during different phases of the menstrual cycle and, more specifically, to detect impending ovulation. T_cwas measured in four women who ingested a series of disposable temperature sensors. Data were collected each minute for 2–7 days and analyzed in 36-h segments by automated cosinor analysis to determine the mesor (mean temperature), amplitude, period, acrophase (time of peak temperature), and predicted circadian minimum core temperature (T_c-min) for each cycle. The T_cmesor was higher (P ≤ 0.001) in the luteal (L) phase (37.39 ±0.13°C) and lower in the preovulatory (P) phase (36.91 ±0.11°C) compared with the follicular (F) phase (37.08 ±0.13°C). The predicted T_c-min was also greater in L (37.06 ± 0.14°C) than in menses (M; 36.69 ± 0.13°C), F (36.6 ± 0.16°C), and P (36.38 ± 0.08°C) (P ≤ 0.0001). During P, the predicted T_c-min was significantly decreased compared with M and F (P ≤ 0.0001). The amplitude of the T_c rhythm was significantly reduced in L compared with all other phases (P ≤ 0.005). Neither the period nor acrophase was affected by menstrual cycle phase in ambulatory subjects. The use of an ingestible temperature sensor in conjunction with fast and accurate cosinor analysis provides a noninvasive method to mark menstrual phases, including the critical preovulatory period.