Nonylphenols (NP) can be detected in the environment worldwide, the major component of the technical NP mixture, p353-NP, produce embryonic misdevelopment in zebrafish (Danio re-rio). Phenotypes with comparable appearance were generated with knock out or other mutants in zebrafish embryos. The genes involved in these studies were: no tail (ntl) gene, spade tail (spt) gene and tbox 6 (tbx6)gene. Aim of this study was to correlate the observed teratogenic effect of p353-NP in zebrafish with expression analysis of genes of the T-Box family involved in normal development of the tail and therefore increase the understanding of the mechanisms by which NP-induced disease. Zebrafish embryos were treated with sub lethal concentrations of different NP isomers. A unique phenotype with massively swollen tail tip was only seen using p353-NP. Expression of ntl gene and spt gene measured by real time PCR was unaltered while expression of tbx6 massively increased after p353-NP administration.