Monthly sampling for the sea star Archaster angulatus revealed at least four cohorts with linear growth in the 0+ and 1+ year groups to about 26 months after which growth slows and conforms to the von Bertalanffy model of growth. The appearance of small individuals (arm radius (R) < 20 mm) from March to May is consistent with the discrete December/January spawning period. Sea stars attain R of 38.7 mm at one year, 73.7 mm at two years and maturity occurs in the second year. The 3+ year and 4+ year cohorts were difficult to detect reliably, but sample number peaks at 90–94 mm and 105–107 mm in January of both 2010 and 2011 are indicative of size at three and four years. Two methods of estimating von Bertalanffy growth parameters both yielded estimates of 127 mm for Linf and k values of 0.41 and 0.49. Comparison of size at age estimates from growth models and cohort analysis suggested the actual value of k after maturity lies between 0.35 and 0.45. Four-armed (1.48%), six-armed (1.26%) and seven-armed (0.11%) individuals were present in the population and arm damage, thought to have resulted from sublethal predation was observed in 13.8% of sea stars.