Examination of the phenomenon of nozzle blockage, encountered during casting of aluminum-killed steel, resulted in the following conclusions: 1. The mechanism of clogging is a complex process involving the precipitation of alumina in the bore of the nozzle augmented by dendritic growth of entrapped steel. 2. At low aluminum concentrations (<0.01% Al soluble), the alumina branches were found to be embedded in a matrix of MnAlSi-oxides, while at higher concentrations (≥0.01% Al soluble) the matrix tended to disappear with the clog consisting only of alumina and frozen steel. 3. Although the tendency for build-up to occur was found even at low aluminum concentrations, complete blockage of the nozzle was encountered only at concentrations ≥ 0.008% Al soluble. 4. While reducing reoxidation of the steel in the tundish through complete shielding decreased the rate of clogging, it did not completely eliminate the problem. 5. A new nozzle design with improved heat transfer characteristics eliminated blockage under strictly controlled operation. However, the stringent requirements for high tundish preheat ≥ 2450 °F) and a minimum steel temperature of 2800 °F, together with the unresolved problem of canting of the stream, are believed to impose serious limitations under actual plant conditions. 6. The concept of a variable orifice for casting achieved by an oversized nozzle with stopper rod control represents a solution to the problem of nozzle blockage. However, application of this technique to the casting of high quality billets depends on the significance of the following: (a) the practicability of a shrouded tundish stream; (b) development of a control system to prevent large variations in casting rate: and (c) the effect of agglomerated alumina, washed out of the nozzle, on product quality.