Comstock Resources (CRK) had the highest return in August 2015 by a mid-cap US stock, returning 134.6%.

ASSET	MONTH	% RETURN
Comstock Resources (CRK)	August 2015	134.61%
Seabridge Gold (SA)	August 2015	60.8%
Cleveland-Cliffs (CLF)	August 2015	58.17%
Central Garden & Pet (CENT)	August 2015	33.99%
Northern Oil & Gas (NOG)	August 2015	30.52%
Extreme Networks (EXTR)	August 2015	29.87%
Endeavour Silver (EXK)	August 2015	26.12%
NovaGold Resources (NG)	August 2015	23.92%
Planet Fitness (PLNT)	August 2015	22.83%
Agilysys (AGYS)	August 2015	22.12%
Post (POST)	August 2015	21.32%
Paycom Software (PAYC)	August 2015	20.4%
Embotelladora Andina (AKO.B)	August 2015	20.18%
Embotelladora Andina (AKO.A)	August 2015	18.02%
Perdoceo Education (PRDO)	August 2015	16.67%
Oshkosh (OSK)	August 2015	16.05%
Signet Jewelers (SIG)	August 2015	13.76%
ICU Medical (ICUI)	August 2015	13.52%
Coty (COTY)	August 2015	13.44%
SSR Mining (SSRM)	August 2015	13.16%
Mercury Systems (MRCY)	August 2015	12.9%
B2Gold (BTG)	August 2015	12.26%
Caesars Entertainment (CZR)	August 2015	11.88%
USA Compression Partners (USAC)	August 2015	11.67%
Black Stone Minerals (BSM)	August 2015	11.67%

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