This afternoon, after the close of trading, S&P announced that it was removing Mesa Air Group from the S&P SmallCap 600 Index. Aftermarket Technology Corp. (Nasdaq: ATAC) will replace Mesa Air Group.

As those of you who are market watchers know — being included in an index encourages institutional buyers to buy a stock — as institutional buyers attempt to mimic the underlying ownership mix of a particular index fund. When S&P selects a stock for inclusion in a fund, institutional investors usually flock to the stock, as they attempt to mimic the underlying stocks in a particular index.

Conversely, when a stock is bumped from an index, you tend to see institutions bail out of the stock. We’ll see what happens tomorrow.

Meanwhile, Mesa had yet another rough day today on Wall Street, as shares ended the day down 13%, closing at 1.23. The stock collapsed last week — losing 41% of its value, closing Friday at 1.41. Trading in Mesa shares was extremely heavy again today.

S&P noted in its release this afternoon that Mesa‘s market cap is now down to $33 million.

Ticker: (Nasdaq: MESA)