Experience with a number of teachers has shown that when they became aware of the uses of the categorizing processes, their teaching became more focused, they took account of student prior experience to a greater extent, and they allowed more time for students to practice the language needed to complete a task. A study was conducted to determine what three groups of middle school teachers knew about some aspects of the categorizing process and also to see how well they could write category labels for 10 very different sets of items. The three groups were sixth grade teachers, seventh and eighth grade language arts teachers, and science teachers from all three grades. A total of 84 teachers participated in the study. As might be expected, the science teachers scored significantly higher than the other two groups on the categorizing task. Surprisingly, however, they also scored significantly higher on the category labeling task, even though only one of the ten groups of items had anything to do with science and even though item analysis showed no pattern of item difficulty for any of the groups. Even more striking was the proportion of science teachers (43%) scoring above 60% on the labeling task as compared to the other two groups who were responsible for teaching language arts to middle school children. By the middle grades, language is an inextricable part of the grouping process by which people order information. If teachers themselves cannot produce good category labels, there is little hope that their students will learn how to use language to codify information and thus remember it, or recognize how others have done so. (FL)