It has been debated whether negative number concepts are cognitively represented on the same mental number line as positive number concepts. The present study reviews this debate and identifies limitations of previous studies. A method with nonspatial stimuli and responses is applied to overcome these limitations and to document a systematic implicit association of negative numbers with left space, thus indicating a leftward extension of the mental number line. Importantly, this result only held for left-to-right counting adults. Implications for the experiential basis of abstract conceptual knowledge are discussed.