We have made pulsar microstructure observations of PSR 1133+16 at the Arecibo Observatory. Cross-correlation of linearly polarized microstructure between 111 MHz and 318 MHz results in a value of the dispersion measure of DM = 4.8462 ± 0.0004 pc cm–3 at epoch 1989.9. This value is consistent with the average profile determination of Phillips and Wolszczan (1990) obtained at epoch 1989.2, but not with the microstructure value found by Popov et al. (1987) in early 1980. Time variation of the integrated electron content over this ten-year span is a plausible explanation for the discrepancy.