Shares in Tetra Tech Inc. dropped nearly 6 percent Thursday after shares in the company were downgraded by the Stanford Group Co.

Analyst Francesca McCann downgraded the shares from a “buy” to a “hold” citing valuation.

“Tetra Tech remains one of our favorite companies in the water space and we are confident in the long-term growth prospects,” she wrote in a research report Thursday. “However, following the recent run we believe that [Tetra Tech] is fairly valued.”

The Pasadena-based water infrastructure and alternative energy systems designer has seen shares gain more than 50 percent so far this year after posting strong second quarter earnings in April and announcing a $30 million wind-power deal in May.

Shares were down 5.8 percent to $25.20 in early trading Thursday.