The crustose lichens of the genus Rhizocarpon are among the most widely distributed species on rock surfaces worldwide (Armstrong 2011, 2016). Thalli of this species are characterized by yellow-green lichenized areolae on the surface of a non-lichenized fungal hypotha- llus, which forms a dark marginal ring. Rhizocarpon species typically grow very slowly and reach a considerable age, characteristics that have been exploited in numerous studies by earth scientists and archaeologists using these lichens as a tool to estimate the time of exposure of stone surfaces in natural systems (e.g. retreating glaciers, Rodbell (1992)) or to date human artefacts (e.g. tombstones or stone walls, Emerman et al. (2016)). There are hundreds of studies on growth in Rhizocarpon species (McCarthy & Henry 2012), although the large majority of them do not directly measure growth via repeated observations but rather use ‘growth curves’ that are constructed from independently dated substrata. This practice has evoked substantial criticism (e.g. Osborn et al. 2015). In spite of the large number of publications and the occurrence of Rhizocarpon species in both hemispheres over a large latitudinal range from the equator to both the Arctic and Antarctic, there are very few reports about this lichen from the tropics (Rodbell 1992; Solomina et al. 2007; Jomelli et al. 2008). Moreover, none of them deter- mined growth directly, but used the ‘growth curves’ criticized by Osborn et al. (2015) and others. The database is not much better for other crustose lichens. To my knowledge, the only study measuring radial growth in a crustose lichen in the tropics (Zotz 1999) is on an unidentified Cryptothecia species (Arthoniaceae) growing epiphytically in the moist lowland forest of Barro Colorado Island in Panama and this reported remarkably high radial growth rates of >5 mm y − 1. The limited information on the growth of tropical lichens in general, and Rhizocarpon geographicum (L.) DC. in particular, motivated the current study. Twice within three years, I visited the summit region of the Volcán Barú massif in the Republic of Panama. Taking digital photographs of Rhizocarpon thalli on large boulders allowed the direct determina- tion of growth without any of the problematic assumptions that typically underlie licheno- metry (Osborn et al. 2015)