The route heads northwest out of Marion, traversing a rural area and skirting the northeast side of the Union Wildlife Management Area. Its northern terminus at LA 549 is located 1.08 miles (1.74 km) south of the Arkansas state line. LA 551 is an undivided two-lane highway for its entire length.
The western terminus at US 165 and LA 2 is located on the Ouachita–Morehouse parish line. From this point, the route heads eastward through Ouachita Parish for 2.6 miles (4.2 km) before crossing Little Bayou Boeuf into Morehouse Parish. LA 554 intersects LA 139, which connects to Monroe and Bastrop. It continues eastward then makes a sharp curve to the south near its eastern terminus at LA 138. LA 554 is an undivided two-lane highway for its entire length.
The route heads northward from US 84 and follows Brushy Bayou for about 3.0 miles (4.8 km). It then curves slightly northeast to a point on the Tensas River known as Dunbarton. LA 566 turns east and roughly follows the river into the town of Clayton. Here, it intersects the concurrent US 425 and LA 15 at the foot of a twin bridge across the Tensas River. LA 566 curves to the north with the river and crosses from Concordia Parish into Tensas Parish.
Soon after crossing the parish line, LA 566 intersects LA 571, the route of which follows a semi-circle west of Waterproof. LA 566 turns to the southeast at a second intersection with LA 571 and passes through Troy to a point on US 65 and LA 568 just southwest of Waterproof. LA 566 officially proceeds with LA 568 a short distance further to an intersection with LA 570, but this portion of the route is signed in the field only as LA 568.
The route heads northwest from Collinston and shortly intersects LA 3079. It travels through sparsely populated areas until passing just west of the Morehouse Memorial Airport, entering the city of Bastrop. LA 593 intersects LA 139, and the two highways run concurrently into the center of town, becoming a one-way couplet at the Morehouse General Hospital. A few blocks later, LA 593 intersects the one-way couplet of US 165/US 425/LA 2, ending its concurrency with LA 139 and beginning one with US 425. The one-way couplet ends two blocks to the north, and LA 593 splits from US 425 soon afterward. North of town, LA 593 passes by the Morehouse Country Club, the Georgia Pacific Wildlife Management Area, and Bussy Brake, all to the west of the road. LA 593 loops back to the east and ends at a final junction with US 425.
LA 594 initially heads northeast on Texas Avenue from LA 15, near the latter's intersection with US 165 Business. The highway crosses over I-20 and immediately intersects 18th Street, connecting to a tight diamond interchange with the interstate. LA 594 turns to the east parallel with I-20 and intersects mainline US 165 near the Pecanland Mall. After a few miles, LA 594 reaches Millhaven and turns due north. It intersects US 80 as it passes Ouachita Parish High School and continues north through Swartz to its terminus at LA 139.
LA 594 is an undivided two-lane highway for its entire length.
Louisiana Highway 595 (LA 595) runs 3.41 miles (5.49 km) in a north–south direction from a junction with US 80 and LA 133 in Start to the Richland–Morehouse parish line.
The route heads north from the concurrent US 80 and LA 133 in Start, a small community in northwestern Richland Parish. After curving westward, LA 595 turns north at a T-intersection with a local road. State maintenance ends at a bridge over Bayou Lafourche, which forms the boundary between Richland Parish and Morehouse Parish. LA 595 is an undivided two-lane highway for its entire length.