Tetraploid loach (Misgurnus anguillicaudatus) exhibits superior growth performance but delayed reproductive development when compared with diploid individuals. To understand the potential molecular mechanism of this physiological difference between-ploidy,the full-length cDNA and 5′-flanking sequence of cyp19a1a in loach were cloned and analyzed,and the expression of cyp19a1a between-ploidy were compared using qRT-PCR method. The results showed that,the full-length cDNA of cyp19a1a was 1 905 bp,which contained a 29 bp 5′UTR,301 bp 3′UTR and 1 575 bp ORF. Some important functional domains were predicted in the cyp19a1a protein,such as I-helix region,aromatase specific conservative region and heme-binding region. Using hiTAIL-PCR method,a 2 040 bp 5′-flanking sequence was cloned,and the typical function element TATA-box,as well as some important transcription factor binding sites such as SP1,AP-1,C/EBPβ,SRY,ER,CREB,GR were predicted. At the age of 12 month,the body length and weight of tetraploids were significantly higher than those of diploids loach (P<0.05). However,the gonadal development of tetraploids was delayed obviously. In both diploids and tetraploids,the cyp19a1a expression was the highest in the gonad,while cyp19a1b was the highest in the brain. The loach cyp19a1a exhibited higher expression in tetraploids than in diploids in all tissues except for the testis. The cyp19a1b expression in the brain was significantly higher in tetraploid female than in diploid female,while an opposite result was observed in male (P<0.05). Combining all results obtained in the present study,it was suggested that the higher expression of cyp19a1a in tetraploids should be due to the special developmental stage of the gonads,and the delayed sexual maturity in tetraploids may partially contribute to its superior growth performance. In addition,as cyp19a1a and estrogen also regulate fish growth through the GH-IGF pathway,it is speculated that the differential expression of cyp19a1a between-ploidy might be related to the different growth performance and reproductive development of diploid and tetraploid loach.