Attenuation of coda and Lg waves was estimated from events recorded by the seismic network of northern Baja California, Red Sismica del Noroeste de Mexico (RESNOM), in the frequency band 1 to 12 Hz in order to find a regional variation of Qc (crustal quality factor for S waves) in northern Baja California, Mexico. We used the single scattering model of Sato <1977> in the time domain to estimate quality factor Qc and a spectral approach to relate acceleration spectra with distance to estimate the anelastic attenuation coefficient &sgr; of Lg waves. We calculated Qc for short (<10 s) and long lapse times (<70 s) using local events and the events that originated at the main tectonic features of the area. Assuming a frequency dependence of Qc of the form Qc(f )=Qcfα we observed for short lapse times a regional variation of Q0 with low values from about 53 in the Inner Continental Borderland (ICB) province to about 115 in the Peninsular Ranges (PR) and 144 in the Gulf of California Depression (GCD) province. The α values range from 0.70 and 0.85 for events in the ICB and PR to 0.44 in the GCD region. For long lapse times and for events located in the PR we found Q0 values from 49 at stations within the ICB to 101 at stations within the GCD. Attenuation coefficient &ggr; of Lg waves between 1 and 6 Hz was also estimated using events located in southern California with source-station distances up to 420 km. We found that frequency dependence of &ggr; was higher for source-receiver paths going from a typical continental structure to a pull-apart basin (the GCD province) than for paths within the PR. Using all data in a linear inversion process and assuming a group velocity (VLg) of 3.4 km/s, we obtained a power law dependence of the form QLg(f )=288¿20f0.32¿0.02.¿ 1997 American Geophysical Union