Duolingo Inc shares added more than 8% on the inclusion of the stock in the S&P Midcap 400.
The language learning app will join the index effective before the opening of trading on Monday, April 22.
It replaces Cable One, which is moving to the S&P SmallCap 600.
Cable One, in turn, is replacing MDC Holdings in this index.
Shares of Duolingo gained 8.5% at $211 in early trade on Thursday, while Cable One shares had added 4.3% at $410.