Fermented infusions of organic matter were tested for their effects on Culex tarsalis oviposition. Bermuda grass infusion and polluted water collected from a natural oviposition site (La Brea tar pits, CA) enhanced oviposition rates, but an alfalfa infusion and water from a 2nd natural oviposition site (Prado Basin, CA) did not. Bermuda grass infusion was fractionated by dialysis and filter sterilization. Crude Bermuda grass infusion, and fractions of the infusion containing large molecules (> 12,000 daltons), particulates, and microorganisms significantly increased oviposition rates compared to distilled water controls. The fraction containing small molecules was no better than a distilled water control, suggesting that small molecules are not involved in oviposition stimulation in this species. However, using the egg raft counting bioassay, the possibility that the small molecules fraction contained oviposition attractants could not be ruled out. Overall, our experiments suggest that results obtained with the egg raft counting bioassay, which has been used frequently to screen for oviposition attractants, should be interpreted with caution. High oviposition rates in this bioassay may be due to responses to factors such as nonvolatile, contact oviposition stimulants rather than to volatile attractants.