This paper reports on the adoption of sustainability practices by a sample of over 800 firms in New Zealand, contrasting results from members of the Sustainable Business Network (SBN) with non-members. The study also examined differences related to business size. Not surprisingly, large firms were found to be more actively engaged in sustainability. On the other hand, of the sample approximately one-third cited the existence of barriers to adoption of sustainable practices. Of these, larger companies were more likely to state that costs or other priorities inhibited the adoption of such practices. Generally speaking members of an SBN were more likely than non-members to adopt environmental initiatives, citing in particular the importance of reputation and brand, but in terms of total waste and energy waste there were no significant differences between members and non-members. There were few differences between the two groups on the adoption of social practices. In assessing membership of an SBN and company size as determinants of good environmental practices, company size was found to be the more significant factor.