Fries directed the Michigan English Language Institute in its heyday as a model for foreign language teaching; his linguistic interests lay in improving the teaching of English, first to native speakers then as a foreign language. He was a founding member of the Linguistics Society of America, with Bloomfield, and co-author with Robert Lado of a number of influential foreign language teaching manuals.
His influential teaching manuals espoused a strong version of the contrastive analysis hypothesis:
The most efficient materials are those that are based upon a scientific description of the language to be learned, carefully compared with a parallel description of the native language of the learner.
Contrastive analysis was allied with behaviourist psychology in the famed Michigan approach, whose logic he outlines in the preface to the 1957 manual thus:
The 'grammar' lessons here set forth . . . consist basically of exercises to develop habits... The habits to be learned consist of patterns or molds in which the 'words' must be grasped . . . 'Knowing' this grammar for practical use means being able to produce and respond to these signals of structural meaning. To develop such habits efficiently demands practice and more practice, especially oral practice.
(cited in Oller, 1987)
Read about the English Language Institute at the University of Michigan, and the contribution of Charles C. Fries.