The Coldbrook Group of southern New Brunswick is composed almost entirely of volcanic rocks and has been assigned a Precambrian age on the basis of field relationships. Rocks of the group are overlain by fossiliferous Lower Cambrian beds of the Saint John Group.Rubidium-strontium total-rock analyses of 46 samples of Coldbrook Group volcanic rocks have been carried out. Analysis of the data indicates the probable presence of two different isochron ages. One of these is apparently defined by those rocks in which the ratio 87Rb/86Sr is low, with values less than about 1.0. This isochron yields an age of 750 ± 80 million years, with an initial ratio 87Sr/86Sr of 0.7054 ± 0.0010. The other isochron is defined by rocks in which the value of the ratio 87Rb/86Sr is higher, with values greater than about 1.0. The age calculated from the second isochron is 370 ± 38 million years, with an indicated initial value for the ratio 87Sr/86Sr of 0.7135 ± 0.0010.The 750 million year age is interpreted to represent the time of extrusion of the Coldbrook volcanics. The 370 million year age appears to be secondary and related to metamorphism of the Coldbrook Group. This age is correlated with the Acadian orogeny, which strongly deformed this part of the northern Appalachians in Middle to Late Devonian time.