A transient high expression of heat shock protein 70 (HSP 70) in zebrafish Danio rerio at 84 h post fertilization (hpf) suspected to arise from translational control of heat shock cognate 70 (hsc 70) mRNA has been detected in our previous studies. In this report, we examined the patterns of development-regulated hsc 70 mRNA translation in zebrafish tissues and the possible mechanism of translational control. HSP 70 spontaneously expressed in 84 hpf zebrafish contained HSC 70 (pI 5.5) and stressinduced HSP 70 (pI 5.7) at a 10:1 ratio based on 2-D Western blotting. Whole mount in situ hybridization showed very similar patterns of hsc 70 transcription in the head regions and swim bladders of 60-84 hpf zebrafish, while hsp 70 transcripts appeared only in the lens. As in situ immunohistochemistry detected an intense level of HSC 70 synthesis in the swim bladders of 84 hpf zebrafish, but not in those of 60 hpf zebrafish, HSC 70 produced by up-regulated hsc 70 mRNA translation should be critical to certain stages of swim bladder development. RT-PCR-poly(A) tail (RT-PCR-PAT) assay excluded the possible involvement of poly(A) tail elongation in regulating hsc 70 mRNA translation. Unlike zebrafish hsp 70 mRNA, hsc 70 mRNA lacking a long 5 0 untranslated region (5 0 -UTR) that might provide an internal ribosome entry site displayed a weak translatability in rabbit reticulocyte lysates, suggesting that hsc 70 mRNA translation in zebrafish tissues is controlled by the availability of cap-binding proteins.