Fidelity Flex funds are a lineup of Fidelity mutual funds that have zero expense ratios, and include proprietary active and passive funds. Flex funds are currently available only to certain fee-based accounts offered by Fidelity, like Fidelity Go. Unlike many other mutual funds, the Flex funds do not charge management fees or, with limited exceptions, fund expenses. Instead, a portion of the advisory fee you pay is allocated to access the Flex funds in which your account will be invested.