Diversity, equitability, and classification of benthic foraminiferal assemblages in Tarakohe Mudstone (Golden Bay district, western Nelson, New Zealand) are investigated for paleoenvironmental data. A north-west trending basinal biotope, bordered to east and west by a marginal biotope, is recognized in the lower beds of the formation (Hutchinsonian-Altonian Stages). Upper beds (Clifdenian-Lillburnian Stages) appear to have been deposited in a shallow, near-shore environment. The formation represents a phase of marine retreat, probably northward.