Applied Opt (AAOI) had the highest return in July 2017 by a mid-cap US stock, returning 56.2%.

ASSET	MONTH	% RETURN
Applied Opt (AAOI)	July 2017	56.21%
Light & Wonder (LNW)	July 2017	40.87%
Skeena Resources (SKE)	July 2017	38.71%
Hudbay Minerals (HBM)	July 2017	35.65%
Constellium (CSTM)	July 2017	28.47%
Amicus Therapeutics (FOLD)	July 2017	28.09%
TAL Education (TAL)	July 2017	27.56%
Uniqure (QURE)	July 2017	27.5%
Dillard's (DDS)	July 2017	27.43%
Warrior Met Coal (HCC)	July 2017	27.32%
Indivior PLC Ordinary Shares (INDV)	July 2017	25%
Skyline (SKY)	July 2017	24.5%
Chemours (CC)	July 2017	23.66%
Avista (AVA)	July 2017	23.56%
Burford Capital (BUR)	July 2017	23.32%
Tidewater (TDW)	July 2017	23.02%
Sharplink Gaming (SBET)	July 2017	22.64%
MKS Instruments (MKSI)	July 2017	22.47%
Amneal Pharmaceuticals, Inc Class A Common Stock (AMRX)	July 2017	20.19%
Power Solutions International, Inc Common Stock (PSIX)	July 2017	19.76%
Denison Mines (DNN)	July 2017	18.6%
Boston Beer (SAM)	July 2017	18.43%
Axos Financial (AX)	July 2017	17.1%
Brinks (BCO)	July 2017	16.02%
Cytokinetics (CYTK)	July 2017	15.64%

Mid-cap stocks are treated as having a market cap between $2B and $10B.