It showed in a fine partnership with number three Bohannon passing 5,000 first-class runs for Lancashire on his way to a half-century off 96 balls.
But from nowhere, he was lbw to an attempted hoick across the line from the off-spin of Ollie Price to leave the hosts 121-3.
If Gloucestershire sensed an opening, Harris shut that down as he began to dominate proceedings.
While he took 106 balls to reach his fifty, he required only 70 more to reach his second Lancashire century and first at Old Trafford.
And his third fifty came at more than a run-a-ball as he timed the ball superbly, hitting 15 fours and two sixes by the close.
Hurst had been a fine support in reaching his first half-century of the summer and with his and Harris’ 200-run partnership coming off 277 balls, the 21-year-old wicketkeeper was beginning to accelerate until he was caught behind off Zaman Akhter via the toe of the bat.
Nightwatchman Saqib Mahmood was yorked for four in the same over, as the visitors fought back, but with Harris still at the crease, Lancashire will hope for a big first-innings score to set up the game on the second day.
