SAINT LOUIS, MO., September 20, 1861.

Acting Brigadier General JEFFERSON C. DAVIS,

Commanding at Jefferson City:

SIR: It is reported that Lexington is surrounded by an overwhelming rebel force of 16,000, and that our re-enforcements from Utica and Liberty, under command of Brigadier-General Sturgis, are opposite to Lexington, and prevented from crossing the river by two rebel batteries. To assist Colonel Mulligan and his brave little band of 2,000, General Lane will harass the enemy by sudden attacks upon exposed posts upon his flank and rear, and you will act according to the order of the 18th, and endeavor to break through the enemy's lines. Should you not succeed in effecting a junction with Colonel Mulligan at Lexington, you are to retreat, and take such a position as your own strength and the movements and force of the rebels may render advisable.

Should the whole force of the enemy be concentrated around Lexington, it may be sufficient to retreat to Davis' Creek, or at farthest to Dunksburg; in eighter of which cases a junction with the forces of General Lane, consisting of 2,200 volunteers, with a large Home Guard force, may be effected.

Should thee rebels hold Warrensburg with a larger force than yours, or if reliable information should reach yo that McCulloch is also operating towards Lexington, you will take position at Georgetown or Sedalia.

Should McCulloch operate towards Jefferson City, but not be able to reach that place before I can re-enforce it (and I shall start Monday), it should not detain your forward movements from Georgetown to lexington.

You will keep me constantly informed of your own movements and those of the enemy, and will watch constantly the re-enforcements pouring in over the Pacific Railroad, so that you may direct them immediately to their destination and correct confusion.

J. C. FREMONT,

Major-General, Commanding.

HEADQUARTERS, September 20, 1861.

Colonel JEFFERSON C. DAVIS, Jefferson City:

Can you break through the rebel lines and effect a junction with Mulligan?

ASBOTH.

JEFFERSON CITY, September 20, 1861.

General ASBOTH:

I can drive the rebels to the Osage if I can get to them, but I have no means of transportation here. My boats are up the river with troops I have no teams.

JEFF. C. DAVIS.

JEFFERSON CITY, September 20, 1861.

Send me all the cavalry you can spare. I can furnish them with carbines and ammunition.

JEFF. C. DAVIS,
Acting Brigadier-General.

Major-General FREMONT.