The diversity data presented in this paper are based on recently revised collections of conodonts from outcrops (northern Estonia) and core sections of Estonia and western and northern Latvia. Based on variations in the abundance of taxa, four intervals with different general trends in diversity changes can be recognized in the Ordovician part of the succession: (1) the proavus–crassus zones and (2) crassus–anserinus zones, both intervals forming quite distinct diversity cycles; (3) the variabilis–ventilatus zones, characterized in general by stable diversity values, and (4) the ventilatus Zone–the topmost Ordovician, with a steady increase in diversity up to the ordovicicus Zone, followed by a decline during the End-Ordovician Extinction Event. Diversity was lowest in the earliest and latest Ordovician but reached maximum values at the end of the Early Ordovician radiation of conodonts, in the middle and upper Darriwilian, and just before the start of the End-Ordovician Extinction Event. Four supersequences (transgressive–regressive cycles) are proposed here for the northern Baltic Ordovician succession. The diversity changes recognized in the conodont succession demonstrate general correlation with these supersequences: boundaries between supersequences are characterized by low diversity values; diversity increases more or less rapidly in the lower, transgressive parts of the supersequences and decreases in their upper parts.