Stochasticity from gene expression in single cells is known to drive metabolic heterogeneity at the population-level, which is understood to have important consequences for issues such as microbial drug tolerance and treatment of human diseases like cancer. Experimental methods for probing metabolism in single cells currently lag far behind advancements in single-cell genomics, transcriptomics, and proteomics, which motivates the development of computational techniques to bridge this gap in the systems approach to single-cell biology. In this paper, we present SSA-FBA (stochastic simulation algorithm with flux-balance analysis embedded) as a modelling framework for simulating the stochastic dynamics of metabolism in individual cells. SSA-FBA extends the constraint-based formalism of metabolic network modelling to the single-cell regime, providing a suitable approach to simulation when kinetic information is lacking from models. We also describe an advanced algorithm that significantly improves the efficiency of exact SSA-FBA simulations, which is necessary because of the computational costs associated with stochastic simulation and the observation that approximations can be inaccurate and numerically unstable. As a preliminary case study we apply SSA-FBA to a single-cell model of Mycoplasma pneumoniae, and explore the use of simulation to understand the role of stochasticity in metabolism at the single-cell level.Recent experimental advances are driving a data explosion in systems biology by enabling researchers to profile single cells, including their genome, transcriptome, and proteome [1,2]. Such single-cell measurements can yield information on thousands of individual cells in a single experiment. This can provide insights on intracellular function and the role of intercellular heterogeneity in a variety of biological systems ranging from microbial populations [3,4] to human diseases such as cancer [5,6].Relatively less advanced are methodologies to probe the metabolism of single cells [7,8,9,11,12]. This presents a barrier to studying metabolic reprogramming in tumour biology for example, which is now understood to be a central hallmark of cancer [13,14]. Single-cell metabolism is challenging due to low abundances of many metabolites, compartmentalisation in eukaryotic cells, and the wide diversity of intracellular metabolites that lack regular structure [9]. Moreover, metabolism is more dynamic than many cellular processes such as DNA replication and gene expression, which means attempts to capture the metabolic state of an individual cell is susceptible to perturbation by changes in cellular behaviour and the surrounding environment. Current experimental obstacles to studying single-cell metabolism combined with its fundamental biological importance necessitates the development of computational techniques that infer the metabolism of single cells from other sources, such as single-cell transcriptomic or proteomic data and information about metabolism at the population-level [10].While our current capacity ...