Critical collecting, measurement of sections and taxonomic reassessment through the Climacograptus wilsoni Zone (basal Lower Hartfell Formation) of the southern Scottish graptolite zonal sequence suggests that, although only locally developed, it is an easily recognisable and useful biostratigraphic interval. The North Cliff of Hartfell Spa is defined as type section for both the base of the Lower Hartfell Formation and that of the C. wilsoni Zone. The zonal assemblage is figured and described, with particular emphasis being placed on the distinctive species C. wilsoni which has not been redescribed since the beginning of this century.