Di-n-butyl phthalate (DBP) widely used plasticizer in the plastic industry, affects regulation of the endocrine system and causes toxicity in animals. In the present study, the aim was to study the toxic effects/damages of DBP exposure using Hsp70 levels and histopathological changes in Carp liver and gill. Hsp70 expression levels were assessed as specific biomarker of in vivo ecotoxicological stress. Carp (Cyprinus carpio) were exposed to sub-lethal concentration of DBP (di-n-butyl phthalate, 1 mg/L) for 4, 24 and 96 h. Gill and liver tissues were evaluated histopathologically and RNA quantifications for Hsp70 expression levels were carried out using a two-step real-time RT-PCR. In liver, a rapid but non-significant increase in mRNA levels in the first 4 h was observed. mRNA levels significantly increased up to 2-3 fold after 24 and 96 h (p < 0.05). However, irregular mRNA level changes were also recorded: Gill specific and time-dependent regulation of Hsp70 expression were 4-5 fold inhibition after 4 and 24 h (p < 0.05), then increased up to 4 fold after 96 h (p < 0.05). Histopathological findings support altered transcription results as: Epithelial lifting, hyperplasia, fusion of secondary lamellae, telangiectasis, passive hyperemia and hydropic degeneration. Significant alterations of Hsp70 levels were likely due to a tissue specific response against chemical stress, cellular damage and lesions due to DBP. Carp was found to be a suitable experimental model for toxicology, and Hsp70 mRNA levels are reliable, specific biomarkers.