"Scientific argumentation," defined as the ability to develop and analyze scientific claims, support claims with evidence from investigations of the natural world, and explain and evaluate the reasoning that connects the evidence to the claim, is a critical component of current science standards and is consistent with "Common Core State Standards." Consequently, science teachers, researchers and program evaluators often include the process of scientific argumentation among their objectives for instruction and assessment. This paper describes the development of a valid, reliable and practical classroom measure of scientific argumentation. Based on Toulmin's (2003) theoretical framework of scientific argumentation, the 36-item test creates both a total score and subscores covering the ability to identify claims and "qualifiers" in a claim, the ability to distinguish among a "claim," "fact," "opinion," and "data," the ability to distinguish among "authority," "logic" and "theory" as possible reasons one accepts a claim, and the ability to identify "rebuttals" and "counter-arguments." Evidence for the measures validity and reliability is presented. Scores on the full "Test of Scientific Argumentation" were internally reliable (a = 0.82) and subscore reliability estimates ranged from 0.49 to 0.88. The total score and subscale scores correlated moderately with an existing critical thinking measure.