19Researchers in the life sciences are posting their work to preprint servers at an 20 unprecedented and increasing rate, sharing papers online before (or instead of) 21 publication in peer-reviewed journals. Though the popularity and practical benefits of 22 preprints are driving policy changes at journals and funding organizations, there is little 23 bibliometric data available to measure trends in their usage. Here, we collected and 24 analyzed data on all 37,648 preprints that were uploaded to bioRxiv.org, the largest 25 biology-focused preprint server, in its first five years. We find that preprints on bioRxiv 26 are being read more than ever before (1.1 million downloads in October 2018 alone) 27 and that the rate of preprints being posted has increased to a recent high of more than 28 2,100 per month. We also find that two-thirds of bioRxiv preprints posted in 2016 or 29 earlier were later published in peer-reviewed journals, and that the majority of published 30 preprints appeared in a journal less than six months after being posted. We evaluate 31 which journals have published the most preprints, and find that preprints with more 32 downloads are likely to be published in journals with a higher impact factor. Lastly, we 33 developed Rxivist.org, a website for downloading and interacting programmatically with 34 indexed metadata on bioRxiv preprints. 35 37 full-length research articles. PLOS Biology published 19. Genetics published 23. Cell 38 published 35. BioRxiv had posted more articles than all four-combined-by the end of 39 September 3 (Table S1). 40 3 BioRxiv (pronounced "Bio Archive") is a preprint server, a repository to which 41 researchers can post their papers directly to bypass the months-long turnaround time of 42 the publishing process and share their findings with the community more quickly (Berg 43 et al. 2016). Though the idea of preprints is far from new (Cobb 2017), researchers 44 have become vocally frustrated about the lengthy process of distributing research 45 through the conventional pipelines (Powell 2016), and numerous public laments have 46 been published decrying increasingly impractical demands of journals and reviewers 47 53 Against this backdrop, preprints have become a steady source of the most recent 54 research in biology, providing a valuable way to learn about exciting, relevant and high-55impact findings-for free-months or years before that research will appear anywhere 56 else, if at all (Kaiser 2017). It's a practice long familiar to physicists, who began 57 submitting preprints to arXiv, one of the earliest preprint servers, in 1991 (Verma 2017). 58