A technique has been developed for using pre-irradiated TLD's to measure temperature exposure. This ''thermal dosimetry'' is based on the loss or ''fade'' of stored TL signal at a rate proportional to temperature such that the total accumulated fade provides a measure of the past accumulated temperature exposure. Requirements for the effective use of this technique are discussed in relation to practical experience gained in using CaSO/sub 4/:Mn to monitor average environmental water temperatures and the thermal exposure of individual fish.