The scientific literature includes a plethora of terms to denote within-species phenotypic diversity (e.g., morphotypes; ecotypes; ecomorphotypes; ecophenotypes; polymorphisms; and life histories). Here we discuss a particular situation in which different terms (ecotypes and life histories) have been used to describe the same within-species diversity of adult Pacific lamprey Entosphenus tridentatus, note an important challenge in using “ecotypes”, and conclude with a recommendation to use “life histories” to describe the ocean- and stream-maturing forms of Pacific lamprey.