Gentle agitation of Ascaris lumbricoides var. suum eggs with a teflon bar magnet resulted in 90% or more of the eggs being hatched within 24 hours. The rate of hatching was independent of pH, temperature, or composition of the hatching medium under the conditions tested. Viable larvae were maintained in medium 199 containing 20% bovine serum for periods of 50 to 60 days at 37[degree]C and up to 100 days at 20 to 26[degree]C. Growth was observed only at 37[degree]C and was accompanied by a significant change in the internal structure of the larvae and by exsheathing.