Based on the results of a Phase 1 investigation into the nature of legal writing, a prototype writing assessment, the Diagnostic Writing Skills Test (DWST) for entering law students was developed. The DWST is composed of two multiple-choice testlets based on prompts and responses to the Law School Admission Test (LSAT) Writing Sample. It contains a total of 48 items. The DWST was administered to more than 500 entering law students in 5 law schools, and data were collected on the performance of these students in first-year legal writing courses and in law school courses generally. The pretest data indicated that the prototype DWST contained six items that needed revision because of the low percentages answering them correctly, low biserial correlations, or low percentages of a high-scoring group answering them correctly. The reliability of the DWST was estimated in the range of 0.70-0.81. This relatively low reliability was attributed to the local dependence of the testlets, possible multidimensionality, the use of a four-choice item structure, lack of motivation among examinees, and the six questionable items, which have since been revised. Despite these problems, the DWST predicted performance in legal writing courses better than either the LSAT or undergraduate grade-point average. The prototype test was second only to the LSAT in the prediction of law school grade point average, and it made a significant contribution to the prediction of law school writing course grade and law school grade average when used along with the LSAT and undergraduate grade point average in the prediction. Six appendixes contain the instrument in pretest and revised forms and supplemental information. (Contains 14 tables and 20 references.) (Author/SLD)