We describe a 430‐MHz SQUID magnetometer in which both the resonant frequency of the SQUID tank circuit and the input/output impedances of the following 4.2 K GaAs FET amplifier are varactor controlled. The intrinsic energy sensitivity of the magnetometer, in full flux lock from dc to 5 MHz, is 6.8×10−32J/Hz (≡4×10−6&Fgr;0/&sqrt;Hz flux sensitivity). The figure of merit is 2.1×10−31J/Hz.