The SPDR S&P Biotech ETF (NYSEArca: XBI), the second-largest biotech ETF by assets, and theMarket Vectors Biotech ETF (NYSEArca: BBH) gained 11.7% and 11%, respectively, last month. The iShares Nasdaq Biotechnology ETF (NasdaqGS: IBB), the largest biotech ETF, added 10.5%. Like BBH, IBB is heavily allocated to the largest biotech names such as Amegen (NasdaqGS: AMGN) and Biogen (NasdaqGS: BIIB) while XBI is an equal-weight ETF with more exposure to smaller biotech firms. [About That Biotech ETF Bubble]

Even with the August gains, some investors are cautiously approaching biotech ETFs. Combined, IBB, FBT, BBH and XBI added about $180 million in new assets last month, a fraction of the $752.8 million hauled in by the Health Care Select Sector SPDR (NYSEArca: XLV).

Although the quartet finished slightly lower, BBH, FBT, IBB and PBE all made new all-time highs again on Monday.

iShares Nasdaq Biotechnology ETF

ETF Trends editorial team contributed to this post.