This paper considers testing for seasonal unit roots in a univariate time-series process. In the seminal paper in the literature, Hylleberg et al. (1990) [HEGY] develop separate regression-based t- and F-tests for unit roots at the zero, Nyquist and annual (harmonic) frequencies in the context of quarterly data. Recently, Smith et al. (2009) have generalised this approach to allow for an arbitrary seasonal aspect, while Rodrigues and Taylor (2007) develop near-efficient versions of the HEGY tests. Other important extensions of the basic HEGY approach appear in, inter alia, Ghysels et al. (1994), who allow for joint testing across different frequencies, Smith and Taylor (1998), who extend the range of deterministic specifications allowed in HEGY and provide limiting null distributions for the original HEGY statistics, and Rodrigues and Taylor (2004) who develop expressions for the asymptotic local power of the HEGY tests.