Memory representations of spatial information require the choice of one or more reference systems to specify spatial relations. In two experiments we investigated the role of different reference systems for the encoding of spatial information in human memory. In Experiment 1, participants had to reproduce the location of a previously seen dot in relation to two landmarks on a computer screen. The placement of the two landmarks was varied so that they were horizontally or vertically aligned in half of the trials, and diagonally aligned in the other half of the trials. Reproductions showed a similar pattern of distortions for all four different orientations of the landmarks, indicating the use of the landmarks as an allocentric reference system. In Experiment 2, the influence of this allocentric reference system for very brief retention intervals (100 and 400 ms) was demonstrated in a visual discrimination task, extending previous work. The results suggest that landmark-based spatial reference systems are functional within 100 ms of stimulus presentation for most of the observers. Allocentric reference sytems therefore are an essential part even of early mental representations of spatial information.