A study was undertaken to create and validate a diagnostic, task-based Informal Reading Readiness Inventory (IRRI). IRRI subtests were created to reflect four areas found to be important in reading readiness: awareness of self and media, language experience, reasoning, and phonics. Prereading curriculum components formed the basis for test item selection and hence provide the teacher functional information concerning demonstrated student achievement. The IRRI can be administered individually in less than 20 minutes or in two time blocks: ten minutes small group/ten minutes individual. Forty students were selected from five first grades in two central Illinois schools to take the test in September 1978. A stratified random sampling procedure, based on school, ethnicity, and sex was used to select students whose scores on the Peabody Picture Vocabulary test ranged from 90 to 110. In May 1979, 36 students were tested using the California Achievement Test (CAT). IRRI scores significantly correlated with reading achievement scores on the CAT and no significant difference in group means for sex, school, or ethnicity was found. The Informal Reading Readiness Inventory appears to be a highly significant predictor of first grade reading achievement for mid-IQ students. (MKM)