The upper Chazyan (Llandeilian) Cobbs Arm Formation of New World Island, north-central Newfoundland, consists of pure limestones and limestones with varying amounts of volcaniclastic admixture interbedded with calcareous tuff. The depositional environment for the Cobbs Arm Formation was that of a volcanic island arc waning in volcanic activity. A rich and varied conodontophorid fauna has been identified from the limestones. Conodontophorid diversity is low in the limestones with high volcaniclastic admixtures; these were probably deposited in a nearshore, but not necessarily very shallow, environment. Farther offshore where there was less or no admixture of volcanic material the diversity is found to be the highest. Samples from environments of very high wave or current energy show very low diversity or are entirely lacking in conodonts. Distributions of individual conodontophorid species show repetitive patterns with regard to co-occurrences, reciprocal relationships, mutually exclusive occurrences, and correlations with sediment types. The paleoecology of the following species is discussed: Baltoniodus prevariabilis–B. variabilis transition, B. variabilis, Eoplacognathus lindstroemi, E. robustus, Panderodus gracilis, Periodon aculeatus, Protopanderodus varicostatus, Pygodus anserinus, P. serrus, Strachanognathus parva, Walliserodus ethingtoni, and W. nakholmensis. The biostratigraphic implications with particular regard to P. serrus and P. anserinus are discussed and it is concluded that the P. serrus–P. anserinus transition (speciation event) probably has less chronostratigraphic value than previously assumed.