An ultralow noise transimpedance amplifier with a gain of 1010, a bandwidth greater than 10 kHz, and an input equivalent noise power spectral density of 1.7×10-30 A2/Hz (single sided) for frequencies less than 10 Hz and a level of 5×10-30 A2/Hz at 1 kHz is described. This level of performance is achieved with standard electronic devices, rather than batteries, powering the amplifier. Very good agreement between theoretical and experimental noise measurements is achieved due to careful measurement and modeling of the significant noise sources. It is shown that the noise level of the 1010 Ω feedback resistor increases with frequency and contributes a significant level of noise for frequencies in the kHz frequency range. The usefulness of the amplifier is demonstrated through measurement of the noise of a HgCdTe planar photovoltaic 3–5 μm mid-wavelength infrared detector cooled to 80 and 120 K. © 1999 American Institute of Physics.