Two full-length cDNAs of heat shock protein (HSP) genes (Se-hsp90 and Se-hsp70) were cloned from the beet armyworm, Spodoptera exigua, and their expression was investigated in relation to cold shock, heat shock, and development. The open reading frames of Se-hsp90 and Se-hsp70 are 2,154 and 2,004 bp in length, encoding polypeptides of 717 and 667 amino acids with a molecular mass of 82.6 and 72.5 kDa, respectively. Both genes showed high similarity to their counterparts in other species. Transcriptional expression profiles revealed that both genes were significantly up-regulated under thermal stress. However, the temperature at which expression level became significantly higher than that of controls varied between genes. Intensity of response to temperature was more intense for Se-hsp70 than for Se-hsp90, regardless of temperature or developmental stage. However, intensities of response to temperature of either Se-hsp90 or Se-hsp70 varied with developmental stage. The basal expression of both genes was highest in young larvae and decreased with age. Translational expression of Se-Hsp70 was observed by using Western blot, the expression profiles of Se-Hsp70 protein were in high agreement with those of Se-hsp70 RNA under heat or cold stress in larvae and pupae. However, it does not completely accord with that of Se-hsp70 RNA expression during development without thermal stress. These results indicated that, in addition to heat shock responses, both Se-hsp90 and Se-hsp70 might be involved in development.