This is an appeal by the defendant who contends the imposition of costs against him, potentially affecting his gain time, was improper. The state’s response to our order shows the defendant’s offenses oc*660curred prior to July 1, 1985, the effective date of section 27.3455, Florida Statutes (1985). Accordingly, the assessment of costs against appellant violated the ex post facto clause. State v. Yost, 507 So.2d 1099 (Fla. 1987). We reverse and remand with direction that the trial court vacate that part of the sentence which assessed costs against the appellant.