A biexciton in close-packed films of quantum dots can manifest itself in amplified spontaneous emission or, in other words, gain-induced narrowing of the fluorescence emission spectrum [1,2]. Since biexciton lifetime is short (up to 300 ps), we use the transient photoluminescence with excitation below the threshold of the amplified spontaneous emission for detection of biexciton spectrum [3].   We used for investigations highly monodisperse CdSe/CdS/CdZnS “double shell” quantum dots with a mean radius of 2.7 nm. Close-packed quantum dot films (thickness from 14 to 20 m) were fabricated on glass substrates by drop casting from a toluene solution. This film is optically excited from the front face with the passive mode locked laser (539 nm, pulse width 30 ps). The room temperature photoluminescence was collected with lens, dispersed in a polychromator, and detected with a streak camera “Agat-SF” with digital recording. The time resolution of the detection setup is 70 ps, and the spectral resolution is 3 nm. We used different sampling times to select the spectra from the fluorescence track after a single laser shot. The integral action time is 450 ps for spectrum 1, 1000 ps for spectrum 2, 1.9 ns for spectrum 3, and 4.1 ns for spectrum 4 (Fig. 1). The spectrum band at 635.7 nm in the curve 1 disappeared in the final spectrum (curve 4) with 4.1 ns integral action time and band maximum at 628.3 nm. There is no amplified spontaneous emission band in the final spectrum as in [1,2].   The disappearance of the spectral band at 635.7 nm observed in curve 1 in the final spectrum (curve 4), a short lifetime of curve 1 and its red wavelength shift relative to curve 4 make it possible to consider the 635.7 nm band as a biexciton band [4] and estimate the biexciton binding energy of 23 meV.