The AutoFitMode property on the DisplayLayout is similar. There's an option to make the last column fill the available space in the grid.

        private void ultraGrid1_InitializeLayout(object sender, Infragistics.Win.UltraWinGrid.InitializeLayoutEventArgs e)
        {
            UltraGridLayout layout = e.Layout;
            UltraGridOverride ov = layout.Override;

            layout.AutoFitStyle = AutoFitStyle.ExtendLastColumn;
        }

If you don't want the fill column to be the last one, then you could use the ResizeAllColumns and lock the widths of the other columns.

        private void ultraGrid1_InitializeLayout(object sender, Infragistics.Win.UltraWinGrid.InitializeLayoutEventArgs e)
        {
            UltraGridLayout layout = e.Layout;
            UltraGridBand band = layout.Bands[0];
            UltraGridOverride ov = layout.Override;

            layout.AutoFitStyle = AutoFitStyle.ResizeAllColumns;
            foreach (UltraGridColumn column in band.Columns)
            {
                if (column.Key != "String 1")
                {
                    column.MinWidth = column.MaxWidth = column.Width;
                }
            }
        }