By Josh Beckerman

Shares of medical device company Zynex Inc. rose 8% to $23.05 after hours as the company was selected to join the S&P SmallCap 600 index.

S&P Dow Jones Indices said Zynex will replace Kemet Corp. next Tuesday, before trading opens. Kemet is being sold to Yageo Corp.

Zynex said in April that first-quarter revenue rose 66% to $15.2 million.

Write to Josh Beckerman at josh.beckerman@wsj.com