Temperature effects on soil microbiological activity were well-documented in the first half of the 20^th century, and many workers recognized that most (mesophyllic) soil microbes became relatively inactive at temperatures below 5° to 10° C. The term biologic zero first was published in the soil science literature in the first edition of Soil Taxonomy, where it was described as the temperature (5° C) below which biological activity is sufficiently low that reducing conditions do not readily develop in saturated soils. While acknowledged as a generalization and simplification, this microbiologically based concept has continued to be affirmed by numerous workers in soil and wetland science. In the last two decades, the term has worked its way into numerous regulatory documents, usually associated with the concept or definition of growing season. Most recently, a new definition of biologic zero has been introduced by the National Technical Committee for Hydric Soils (NTCHS), which no longer is related primarily to soil microorganisms but, rather, is tied to higher plants. The new definition binds biologic zero to measurements taken at a specified depth (50 cm) and appears to be driven by an interest to modify and define better the concept of growing season for cooler climates. This change represents a fundamental departure from the long-held concept of biologic zero and may have significant ramifications on wetland regulatory issues.