Experimental studies were carried out on infrared-active phonons in the Aurivillius ferroelectric SrBi2Ta2O9 (SBT), using reflectivity measurements (down to 200 cm(-1)) and transmission measurements (down to 100 cm(-1)) on crystals, pellets, and thin films. An analysis was done of the contrasting consequences of the competing orthorhombic and pseudotetragonal symmetries in SBT. Reflectivity results for light polarized in the ab plane show that the anisotropy in this plane is small in the frequency range from 300 to 1200 cm(-1),, indicating the influence of the approximate tetragonal symmetry. Dielectric dispersion properties were derived for this polarization (E perpendicular to c) in this frequency range. The transverse-optical (TO) and longitudinal-optical (LO) frequencies corresponding to the dominant E perpendicular to c band are 613 and 773 cm(-1), respectively. This strong, high-frequency band arises from a mode dominated by the motion of the oxygen sublattice; its TO and LO frequencies yield an oxygen-ion Szigeti effective charge of -1.5e. Frequency estimates for the TO (LO) pairs of other strong bands are 188(330) and 334(451) cm(-1) for E perpendicular to c, and 610(675) and 780(815) cm(-1) for E parallel to c. In addition to the main infrared bands, the main Raman bands of SBT are also reported.