This paper examines reading achievement when the maternal/paternal language has become a de facto second language. The performance of a cohort of Greek-Australian high school students (N = 270) on a diagnostic Greek reading test was significantly below that of pupils in second to fourth grades in Greece. The mean item difficulty for Greek-Australian high school students was 0.35 compared with 0.51 for second grade, 0.69 for third grade and 0.80 for fourth grade pupils in Greece. The pattern of responding indicated that the Rasch model fitted the data. The effects of background factors were also examined. Students whose mother or father spoke Greek had statistically significant higher levels of ability than those who spoke Greek and English or English alone. It was also found that the length of the key words (correct responses) had a large effect on the difficulty of the questions; the longer the key word, the more difficult the question was. The implications of the results for assessment of reading in and the acquisition of an ethnic or second language are discussed. (Contains 3 tables and 4 figures.)