The dominant radiationles decay channel in crystalline tetracene at 300 °K is a fission of an excited singlet into two triplet excitons with a rate constantγS= 1.5 × 10−12± 5% cm3-sec−1. The efficiency of this process at room temperature is estimated as 95%and constitutesan efficient intersystem crossing mechanism. At light intensities I ⩾ 1015quanta-cm−2sec−1(366 μm excitation), the triplet densities at 300 °K are sufficiently high to produce radiative triplet-triplet annihilation or fusion. As the light intensity is increased the quantum efficiency of fluorescence increases, and eventually reaches a constant value (about twice its value in the low intensity region, where fusion is not important). It is assumed that triplet-triplet fusion gives rise to either an excited singlet (rate constant γTS = (4.8 ± 1.2) × 10−10cm3-sec−1), or excited triplet (rate constant γTT = (11 ± 5) × 10−10cm3-sec−1). The effect of a magnetic field H = 4000 gauss on the rate constants is determined. The radiative triplet-triplet fusion constant and γS both decrease in approximately the same manner when a magnetic field is applied. γTT is shown to be at most slightly dependent on H.