The present paper reports a study on the in vivo diagnosis and egg excretion rhythm of the pinworm Passalurus ambiguus in domesticated rabbits. Three copromicroscopic techniques were compared: the cellophane tape test, the McMaster technique, and the FLOTAC technique. Out of the 51 New Zealand White rabbit does examined, 42 (82.3%) resulted positive when examined with the FLOTAC technique, 39 (76.5%) with the cellophane tape test, and 29 (56.9%) with the McMaster technique. The agreement between the FLOTAC technique and the cellophane tape test was almost perfect (greater than 0.8); only moderate were the agreements (0.4) between the FLOTAC and the McMaster techniques and between the McMaster technique and the cellophane tape test. The results showed that the FLOTAC technique can be used for the quali-quantitative coprological diagnosis of P. ambiguus in rabbits due to its great sensitivity, as already shown for parasites of other animal species. The circadian rhythm of egg excretion by P. ambiguus was studied utilizing 42 individually caged rabbit does; fecal samples were collected from each cage every 6 h, i.e., at 6:00-12:00 hours, 12:00-18:00 hours, 18:00-24:00 hours, and 24:00-6:00 hours, and were analyzed by the FLOTAC technique. A circadian rhythm of P. ambiguus egg excretion was found, with significant lower values at 6:00-12:00 hours. In conclusion, the present study showed that the FLOTAC technique is the best copromicroscopic method for assessing P. ambiguus prevalence and intensity in rabbits and that the afternoon and evening hours are the best times for fecal sampling to perform the pinworm diagnosis.