Photometric and spectroscopic observations of the {beta} Cephei star 16 Lacertae obtained in 1983 and 1984 are presented and analysed. Using all the published maxima we have computed ephemerides for each of the three pulsational modes. No period variation is detected from 1950 to 1983. In order to identify the pulsational modes corresponding to the three main periods of 16 Lac we have applied different methods based on photometric and radial velocity variations. All give the same results: the mode corresponding to the largest amplitude P_1_ (016917d) is radial, P_2_ (017079d) is a nonradial mode of degree l=2. For P_3_ (018171d) the results are more dubious, but P_3_ is probably a l=1 nonradial mode. We have also studied the interaction between pulsation and binarity in 16 Lac. We confirm the existence of a resonance phenomenon between the radial and nonradial modes: the ratios between the beat frequencies of the pulsational modes and the orbital frequency are close but significantly different from simple rational numbers. According to Kato (1974) that means that the nonradial modes might be excited or at least enhanced by tidal effects. They would have been selected among all the possible modes because their ability to be resonant with the radial mode. The photometric amplitude of P_3_ in both filters appears to be variable from night to night in correlation with the orbital phase but despite its rapid and irregular fluctuations, the P_3_ amplitude stayed in constant average over 80 years. The present observations confirm the decrease of P_1_ and P_2_ amplitudes since 1950 but an analysis of older radial velocity data shows that the amplitude of the P_1_ mode went through a maximum around the middle of the century before the observed decrease of the last thirty years. Objects concerned: --------------------------------------------------- RA (2000) DE Name(s) --------------------------------------------------- 22 56 23.5 +41 36 14 16 Lac = HR 8725 = HD 216916 ---------------------------------------------------