Within the extended-Huckel theory we present the influence of carbon 3s orbitals on the band structure of diamond. 3s orbitals are shown to account for the indirect band gap. The results of EHMO-ASED (atom superposition and electron delocalization) calculations agree well with orthogonalized plane-wave (OPW), pseudopotential, and ab initio studies; bond length and bulk modulus are nicely reproduced. For comparison, calculated bond lengths for a few selected hydrocarbons and for C-60 are included.