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A composite far‐infrared bolometer has been constructed that uses an aluminum film at the superconducting transition temperature of 1.3 K as the temperature‐sensitive element. The film is evaporated on one edge of a 4×4‐mm sapphire substrate, which is coated on the reverse side with an absorbing film of bismuth. The best bolometer has an electrical NEP of (1.7±0.1) ×10-15 W Hz-1/2 at 2 Hz, and a specific detectivity D* of (1.1±0.1) ×1014 cm W-1 Hz1/2. This measured electrical NEP is within a factor 2 of the fundamental thermal noise limit.