In spring 2020 the whole world went through the "black swan"-COVID-19 pandemic. The healthcare systems of all countries and the world economy, in general, became very stressed. The extraordinary decline of activity in all spheres, except healthcare, led to a drop in the demand for transport services, including city public transport. It was important for city management to support the sustainability of the local transport system. The article presents fundamental approaches to assessing the sustainability of a transport service, particularly city passenger public transport (CPPT), for the example of the large Russian city Tyumen (size of population-nearly 807 thousand people). Methods of analysis of the sustainability of the transport process in conditions of negative environmental impact (COVID-19 pandemic) are considered. During the period from 30.03.2020 to 31.05.2020 (nine weeks-the acute phase of COVID-19 pandemic) structural sustainability of the CPPT system in Tyumen kept a high level. By changing the parameters of the planned characteristics of the CPPT system state, an attempt to adapt the transport service supply to a sharp decrease in transport demand was made. In the period of "self-isolation", the demand for the CPPT transport service reduced more than the transport service supply. Sustainability of CPPT functioning was evaluated by calculating the elasticity of the transport supply (number of trips) in relation to the actual demand (actual volume of transportations). Calculation of the elasticity index of the CPPT system of Tyumen during nine weeks of April-May 2020 (duration of "self-isolation") is provided. A conclusion was made from the results of the research. In particular, it was found that the foreground target function of city management was the maintenance of a high level of transportation processes to the detriment of the transportations' effectiveness. Such a policy led to contradictory results-the additional financial expenses at the rate of 135-150 million rubles and quite a high level of contentment of the Tyumen population with the quality of the CPPT work (sociological research established that 80-85% of respondents were satisfied with the quality of the transport service in April-May 2020). © 2020 by the authors.