| Population Change 1990-2000 | ||||
| County or Municipality | 2000 Population | 1990 Population | Population Change | Percent Change |
| Pennsylvania | 12 281 054 | 11 881 643 | 399 411 | 3.4 |
| Clarion County | 41 765 | 41 699 | 66 | 0.2 |
| Ashland township | 1 081 | 1 019 | 62 | 6.1 |
| Beaver township | 1 753 | 1 840 | - 87 | - 4.7 |
| Brady township | 62 | 78 | - 16 | - 20.5 |
| Callensburg borough | 224 | 205 | 19 | 9.3 |
| Clarion borough | 6 185 | 6 457 | - 272 | - 4.2 |
| Clarion township | 3 273 | 3 306 | - 33 | - 1.0 |
| East Brady borough | 1 038 | 1 047 | - 9 | - 0.9 |
| Elk township | 1 519 | 1 526 | - 7 | - 0.5 |
| Emlenton borough | 10 | 10 | 0 | 0.0 |
| Farmington township | 1 986 | 1 927 | 59 | 3.1 |
| Foxburg borough | 275 | 262 | 13 | 5.0 |
| Hawthorn borough | 587 | 528 | 59 | 11.2 |
| Highland township | 633 | 573 | 60 | 10.5 |
| Knox borough | 1 176 | 1 182 | - 6 | - 0.5 |
| Knox township | 1 045 | 1 281 | - 236 | - 18.4 |
| Licking township | 479 | 483 | - 4 | - 0.8 |
| Limestone township | 1 773 | 1 686 | 87 | 5.2 |
| Madison township | 1 442 | 1 423 | 19 | 1.3 |
| Millcreek township | 415 | 407 | 8 | 2.0 |
| Monroe township | 1 587 | 1 314 | 273 | 20.8 |
| New Bethlehem borough | 1 057 | 1 151 | - 94 | - 8.2 |
| Paint township | 1 778 | 1 730 | 48 | 2.8 |
| Perry township | 1 064 | 1 076 | - 12 | - 1.1 |
| Piney township | 516 | 515 | 1 | 0.2 |
| Porter township | 1 466 | 1 564 | - 98 | - 6.3 |
| Redbank township | 1 502 | 1 576 | - 74 | - 4.7 |
| Richland township | 553 | 490 | 63 | 12.9 |
| Rimersburg borough | 1 051 | 1 053 | - 2 | - 0.2 |
| St. Petersburg borough | 405 | 349 | 56 | 16.0 |
| Salem township | 852 | 893 | - 41 | - 4.6 |
| Shippenville borough | 505 | 474 | 31 | 6.5 |
| Sligo borough | 728 | 706 | 22 | 3.1 |
| Strattanville borough | 542 | 490 | 52 | 10.6 |
| Toby township | 1 166 | 1 153 | 13 | 1.1 |
| Washington township | 2 037 | 1 925 | 112 | 5.8 |
| Source: U.S. Census Bureau | Prepared by: Pennsylvania State Data Center | |||