The egg output of Trichuris muris was found to reflect the stage of maturity and age of the worms present. During the 2nd week of patency the daily egg output per female worm ranged from 4,000 to 5,000. Maximum output occurred between the 3rd and 5th weeks of patency, with a calculated range of 7,500 to 7,900 eggs per female per day. Late in patency, week 10, the count decreased to a range of 2,100 to 2,400 eggs. The ratio of male to female worms increased as the infection progressed. In early patency the mean ratio was 1.00:1.42; terminally it was 1.00 male to 0.50 or less females. During investigations of the bionomics of Trichuris muris studies were initiated on the daily egg output of the female worms. The egg production of T. trichiura has been variously stated to be from 1,000 to 46,000 eggs per female per day (p. 342, Faust and Russell, 1964). In light of the present knowledge of the self cure or of T. muris at different stages of preand postpatency (Campbell, 1963; Pike, 1963; Wakelin, 1967), the egg output might be expected to vary during the course of patency. This variance would depend upon changes in the male to female worm ratio, worm length, and duration of infection. MATERIALS AND METHODS The strain of T. muris used was kindly provided by Dr. 0. D. Standen of the Wellcome Laboratories, London (for the origin of the culture see Keeling, 1961). Eggs were collected from the feces of infected mice by a modification of the zinc sulfate centrifugal-flotation technique of Faust et al. (1938, 1939). After flotation the eggs were washed in distilled water and transferred to a 40-ml stender dish and suspended in 12 ml of 0.5% formolphysiological saline solution to a depth of 8 mm. The dish was covered and, without further aeration, the eggs were left at room temperature for 38 days, then stored in the refrigerator at 4 C. Before animal inoculation the egg cultures were brought to room temperature; the formol-saline solution was replaced with 2% tragacanth solution. Movement of the ovic larvae was verified microscopically. The culture was placed on a magnetic stirrer to assure homogeneity during aliquot counting and inoculation. The volume was adjusted to Received for publication 24 September 1968. * This study was supported in part by the U. S. Public Health Service Training Grant 2E-55 (C2 and C3). t Present address: Department of Microbiology, New York Medical College, 5th Avenue and 106th Street, New York, New York. yield the desired number of eggs in 0.2 ml which was given each mouse by gavage. Male weanling mice were used. The C57 brown mice were raised in the nematode-free colony maintained by Dr. K. L. Hussey at Columbia University from Rockland Farms stock. The. DBA-2 mice were raised by the author through brother-sister matings of Jackson Memorial Laboratory stock. Beginning on day 28 postinoculation mice were housed in individual cages with hardware-cloth floors raised 0.25 inch from the base of the cage. Purina laboratory chow and water were given ad lib. Daily fecal examinations for eggs were made from day 28 to patency, and thereafter as indicated below. In Experiment I, 14 male C57 brown mice were each given 100 eggs. Four males which became patent on day 32 were used. During the 2nd week of patency the total 24-hr fecal output of each mouse was used for a Stoll egg count. The contents of the Stoll flasks were then filtered through a 40-mesh brass sieve to recover any worms passed. Each daily count was divided by the number of female worms obtained by natural egression and at autopsy on day 45. At autopsy the cecum and large intestine were removed, and the male and female worms counted. After removal the living worms from Experiment I were fixed in 70 C 70% alcohol and measured by camera lucida and a map measure (Rappaport and Wells, 1951). In Experiment II, 34 DBA-2 mice were used, of which 17 each received 150 eggs and 17 each 300 eggs. Beginning week 6 and once weekly until autopsy a Stoll count of each total 24-hr fecal output was done to observe fluctuations in egg deposition. Mice in Experiment II were killed between days 98 and 105 or earlier if patency ceased.