Tissues from two bighorn sheep fetuses and four neonatal bighorn sheep lambs were examined for evidence of natural Protostongylus infection occurring by the transplacental route. Forty-three third-stage larvae (L3) were recovered from the fetal liver of case 1; one L3 from the cotyledons of case 2; 189 L3 from the liver and 46 L3 from the lungs of case 3; 146 La from the liver and 43 L3 from the lungs of case 4; and 133 L3 from the liver and 33 L3 from the lungs of case 5. Fifty-two L3 and fourth-stage larvae (L4) were recovered from the lungs and five L3 from the liver of case 6. Digestion with Pepsin-HCL was a more effective method for recovery of L3 than baermannization, but L4 larvae were digested.
