High frequency trading firm KCG Holdings (KCG) saw trading of shares halted Wednesday afternoon after a bid was made for the company by rival Virtu Financial (VIRT), resulting in KCG stock spiking 23% to close at $16.90. Virtu Financial’s bid valued the company at $1.2 billion.

KCG Holdings stock showed a bit of volatility in after hours, as shares quickly sank 3.6% to $16.30 before surging even higher to $17.50 as of this writing. Meanwhile, shares in Virtu declined to $16.40 in after hours trading before jumping $16.75.

New York-based KCG was founded in 2013 as a high-frequency trading firm that also provides electronic execution, institutional sales and trading and market making. Virtu, meanwhile, was founded a few years earlier in 2008 (by Vincent Viola, initially a Trump nominee for Secretary of the Army), and quickly became one of the largest high-frequency trading firms, operating on a number of exchanges, markets and dark pools.

Virtu Financial’s bid comes at a time when trading firms have largely struggled due to chronic low volatility in markets. The Wall Street Journal reports that KGC is currently mulling the offer with the help of advisers.