A 14-month study was conducted of the polychaetous annelids present on attached wooden blocks replaced monthly and quarterly at nine stations in Los Angeles Harbor in 2013-2014 and compared to the results conducted at the same stations in 1950-1951. Many environmental changes have occurred in the harbors over the past 63 years. The harbor has been expanded into the outer harbor, channels have been deepened and pollution abatement programs initiated. The water quality has been improved as a result of these changes especially in the inner harbor area where the dissolved oxygen in the water was low or absent in 1950-1951 and was over 6.0 mg/L in 2013-2014. The number of polychaetes species in these two studies increased from 23 to 64. The serpulid Hydroides elegans was a dominate species in both studies but the pollution indicator Capitella capitata, common in the earlier study, was rare in the recent study. There was a seasonal occurrence in both the number of species and specimens with the highs in the warmer months and lows during December through March in both studies.