Ovarian dissections were performed on laboratory reared, nulliparous, sugar-fed Culex nigripalpus Theobald females and characterized for parity using both dilatation and tracheation methods. In all females dissected the follicles were characterized as either nulliparous or 'resorbed' 4 days after emergence, but 1 week after emergence 16% to 50% of the females dissected had developed 1 aberrant dilatation, and within the next 2 weeks more than 90% of the females had 1 to 3 aberrant dilatations indicative of the parous condition. In a comparison of the females showing 1-3 aberrant dilatations, the ovarian tracheoles showed some partially uncoiled skeins indicative of the nulliparous condition. Thus no correlation was found between ovarian tracheoles and dilatations in determining parity in Cx. nigripalpus. A comparison with other Culex mosquitoes suggests that ovarian tracheation is more reliable than dilatation method for a determination of parity in Culex species.