Immature stages of the slash pine flower thrips, Gnophothrips fuscus (Morgan) (Thysanoptera: Phlaeothripidae) are described for the first time, based on light and scanning electron microscopy. The chorion of the egg has a reticulate sculpturing anteriorly that may be diagnostic for the species. The other stages differ from one another in antennal structure and chaetotaxy. A key to larvae, prepupae, and pupae is presented. Some morphological and behavioral differences between adults of G. fuscus and the similar-looking Leptothrips pini (Watson) (Thysanoptera: Phlaeothripidae] are given.